They have had many requestes for such a program but, at this time, none are in the pipeline.
Garmin, Tom Tom, most all of the GPS device makers, they all use essentially the same mapping and routing data (except the makers of PC Miler and CoPilot, who continue to insist upon using their own very flawed data). The thing about truck routes, clearances, HAZMAT, things like that, the vast majority of that data is already there, free for the taking, from the US and Canadian governments. Yet, none of these manufacturers or their data suppliers seem interested in utilizing it. I truly do not understand why, particularly since the cost of obtaining the data is miniscule, and it would only serve to sell more GPS units.
If they're not careful, Qualcomm is gonna beat 'em to the punch, putting a huge dent in a potentially huge market.
As for which one to get, that depends more on your price range and personal preferences. Garmin versus Tom Tom is like the difference between Microsoft Money and Quicken, or between Coke and Pepsi. I prefer Garmin, but that's because I like the interface better than the Tom Tom units. In a given price range, a Garmin and Tom Tom will have generally the same features and will perform equally well. A $600 Garmin will blow the doors off a $200 Tom Tom, and visa versa. The both use the same map data, but they do use different routing algorithms, which will give you slightly different routes to take on some trips. The Garmin seems to be slightly more efficient with its routing, but that is more of a persona preference, too.
If have a Garmin c500, which is basically a newer-ish cousin of Terry's c330 (he'll be happy with the map upgrade, BTW). The c550 has a newer SiRF chip that grabs and holds onto the satellite signal a little better, and it has the real-time traffic information and routing (and other features like it's an MP3 player, as well as hands-free calling via Bluetooth), but they are mostly the same unit, since they use the same mapping and routing software. All Garmin units that are running the most current version of the mapping software will perform the same. The difference is the features. The very cheap units will not have maps of Canada, something to take note of. Even if you don't run there, you may one day vacation there.
The new Nuvi units are newer technology, and have many additional features. One really kewl feature of the mid-range priced and above units is the "Where did I park the car?" feature, where you take the unit with you into the mall, and the unit remembers the GPS position of the car was when you removed it, and will take you right to it if you forget where you parked. The Nuvi's are smaller, like an iPod or a deck of cards, and are makde to be portable. They have more "touristy" features, as well (like currency converter, audio book player, jpg picture viewer, games, as well as an MP3 player), so you can go to a strange city and carry it with you for walking navigation to local shops and restaurants. I would have really liked that when I was recently in downtown Boston, actually.
The high end Nuvi's, the 800 series, have speech recognition, so you can tell them what to do instead of punching the screen, if you do desire. Seen it work, and it's killer. The remote button mounts on the steering wheel, and you punch the button and start telling it what to do, like, "Find address" and then start speaking the address. Very kewl.
It also has an FM transmitter so you can hear the voice prompts, MP3's and audio books through the vehicle's stereo speakers. And, it's Bluetooth, so you can make hands-free calls on the cell.
If I were in the market for a new GPS unit today, I'd get either the Nuvi 760 (no voice commands, MSRP of $750) or the Nuvi 850 (MSRP of $850). Anything in the $650 and up range is where you'll find the advanced features (arguably the better units), but even all but the the cheapest Garmin or Tom Tom will still use the same maps and routing as the more expensive units.
It all depends on personal preference, and money.
Google the various units you may be interested in and find the reviews. Read as many as you can find, even the ones that aren't necessarily the ones for the specific model numbers you are interested in. You'll find out more of what other units can and cannot do, giving you a better idea of just what kind of feature you can expect, and demand, out of the one you settle on.