Verdict: After living with both the TomTom GO 910 and the Garmin StreetPilot c550 for 4 weeks, I've decided to retire the 910 and am currently driving exclusively with the Garmin c550.

Garmin c550's Map View

The TomTom 910 has more features than you can shake a stick at, and some of them are even useful features, such as iPod connectivity, traffic updates, bluetooth cell phone integration. Almost every aspect of TomTom's software can be customized. Want a compass on the map? No problem, TomTom lets you add one. Want to have certain things read aloud but not others? You can be quite granular in how much voice prompting the system gives you on the TomTom. Want to hide some of the menus you don't use much? TomTom has a "show fewer options" option. On paper, the TomTom looks like a better choice than the Garmin.

Features the TomTom GO 910 has that the Garmin c550 does not have:

1. Large (4 inch) wide screen display. The Garmin c550 is 2.8 inches wide.
2. Large internal storage (20GB hard drive) -- the Garmin uses solid state memory instead, and only has around 700Mb available for user storage (although it does have an SD card slot for expandability).
3. Best text-to-speech in the industry. It really sounds like a human voice, even when reading street names aloud.
4. Real-time weather data capability
5. iPod connectivity (via an optional cable)
6. Included remote control

So, given TomTom's feature superiority, why am I keeping the c550 and sending the TomTom 910 packing back to the Netherlands? The answer is two-fold:

1. The GO 910 is plagued by a series of problems that prevent it from functioning as advertised/expected
2. The Garmin c550 has fewer features, but what it does have it does better than the 910

Allow me to explain:

First, let's look at the problems the GO 910 has:

1. Terrible Customer Support
   TomTom has quite possible the worst customer support I've ever encountered. Calling the support number lands you in “tier 1” support. These people are qualified to answer only the most basic questions. For anything more significant, or for any hardware return requests, you need to speak to “tier 2”. Tier 2 support cannot be reached directly; instead, they take a message and return your call “in about 48 hours”. When you finally do speak to someone, they frequently give you incorrect information, or no information at all. For example, I called to ask how you save your current location on the GO 910, and was told there is no way to do that (which is completely wrong).
2. TomTom's faulty windshield mount
   TomTom's crummy windshield mount is now a matter of public record. Internet forums are full of people detailing how the mount broke during normal usage. In my own case, after just 5 days of gentle use, the mount started showing signs of wear; the mount was unable to properly support the weight of the 910, and would begin dipping downward until the unit was facing the floor. 3 days later, the mount broke apart on me altogether. When I called TomTom's support number, I was told the windshield mount was a known issue and the reason TomTom wasn't yet selling the 910 directly -- not exactly the behavior of a company looking to treat its customers right
3. Buggy Code
   My TomTom GO 910 shipped with software version 6.01 on it. It wasn't long before I noticed that when driving, my estimated time of arrive would shift 2-3 hours all of a sudden. Investigating the issue revealed that the 910 was changing the current time for no reason at all. Another call into TomTom support revealed this was another known issue, and the rep needed to email me an unreleased firmware update to fix the issue. Shortly thereafter, more issues popped up, and I had to download another firmware release to fix those issues, and so on and so on...
4. Cumbersome Menu System
   You know, a large feature set isn't always a good thing; it can be a double edge sword. In this case, TomTom has made some strange User Interface decisions that make operating the unit cumbersome. For example, the unit ships by default with the "Show fewer menu options" enabled. Sounds great, until you realize that also means you have no way to cancel a route. If you want to clear your current route, you have to first enable the "Show all options" option, and then you will find a "Clear Route" button buried 2 menu levels deep from the main map -- very annoying. Another menu annoyance had to do with the way you input names -- there's no backspace! you have to cancel and start over if you make a mistake. hard to believe, but true.
5. TeleAtlas Mapping Data
   This is perhaps the biggest issue I had with the TomTom GO 910. On three different occasions I found the TeleAtlas maps on the 910 to be either off my as much as .5 miles, or missing addresses altogether. In all three such instances, the Garmin (which uses NAVTEQ mapping data) had the accurate and correct address. From what I've been able to find online, TeleAtlas is an excellent provider of European mapping data. NAVTEQ, however, is superior for North America. No surprise, then, that NAVTEQ is the mapping data behind Google Maps, Yahoo! Maps, MSN Maps, and Mapquest.
6. Poor Traffic Implementation
   The GO 910 requires a compatible Bluetooth enabled cell phone in order to receive real-time traffic data. As of this writing, very few phones are supported. TomTom plans to sell an external RDS traffic receiver antenna, but it is not yet available. The Garmin c550, on the other hand, ships with everything you need to get traffic data right out of the box.

The Garmin StreetPilot c550 suffers from none of the problems listed above. In fact, they have a very good customer service department, an excellently designed windshield mount, and a simple menu system that even my mother could navigate.

Now lets look at things the Garmin StreetPilot c550 does better than the GO 910:

1. NAVTEQ Maps
   The c550 uses NAVTEQ maps, which are superior to TeleAtlas (in North America). This is reason alone to go with the c550 over the TomTom 910. On multiple occasions the TomTom was unable to route me to my destination because of poor mapping data. The NAVTEQ based garmin was able to save the day each time.
2. Stereo Speakers
   The c550 has two excellent speakers that deliver crisp, clear instructions even at high volume. By contrast, the TomTom has only one small speaker that gets distorted at high volumes (which is sometimes necessary in a car traveling on the highway).
3. Mounting Hardware
   In a way I feel bad even mentioning this, since it should just be assumed the mounting hardware will work as designed. However given all the issues around TomTom's 910 mount, it's worth saying the Garmin's mount is solid and well designed.
4. Fast Boot and Shutdown Times
   The Garmin boots up and turns off much faster than the TomTom. I'm assuming this has to do with Garmin's using flash memory instead of a hard disk. Either way, it's nice to press the power button and have the unit almost instantly available to use. Similarly it's nice to be able to turn the unit off without waiting for it to shut down.
5. Better Display
   Yes, the TomTom has a larger display. However I found the TomTom difficult to read in direct bright sunlight. Garmin's screen is bright, clear, and easy to read even in bright sunlight, living up to Garmin's claim of "sunlight readable display".
6. Volume Adjust Knob
   Physical volume control wheel makes it easy to adjust or silence the volume on the c550 (and without having to look at the unit or navigate menus).
7. Better Battery Life
   The Garmin c550 gets approximately twice the battery life out of each charge than the TomTom. The Garmin c550 can operate up to 8 hours on a full charge. TomTom's GO 910 lasts up to 4 hours.
8. 3D perspective
   I found the 3D perspective easier to read on the Garmin than on the TomTom. The angle is slightly different between the two. It's subtle, but I preferred Garmin's approach.

I had only a few complaints with the Garmin, most notably being that there is no signal strength indicator on the map view, and that overall the icons and menus on the Garmin could use a redesign -- they're looking a little Windows 3.1'ish. I'd like to see Garmin freshen the overall software UI up a little, but that's a minor complaint overall.

Conclusion

When using the TomTom GO 910, you get the distinct feeling that the product isn't finished. After calling support, you may even suspect that TomTom is being run out of a garage somewhere and is little more than a two person operation. TomTom is the new kid on the gps block and it shows. They released the 910 before it was adequately debugged, and before the accessories are actually available for it, and before a suitable windshield mount was designed.

Garmin is a mature company with good support and products that work as expected. The bottom line for me is this: the Garmin isn't as elegant as TomTom's GO 910, but I'll take reliability and mapping accuracy in my GPS over features and bigger display any day.