Hi BentJay. Of course there is no one best unit, but you should be able to find the best one for you with a little information.
The first big choice is whether to buy a mapping GPS receiver (GPSR), that is, one that stores maps internally and always shows you where you are on the map. Non-mapping units are much cheaper. They show a course pointer and distance to the next waypoint along the route. The AC waypoints and routes are designed to work with either, but if you want guidance off the route, plan on using paper maps or a mapping GPSR.
In either case, you should plan on taking the good paper maps from AC. The GPS routes complement them, but do not come close to replacing them with all their bike-specific information. And the paper does not require batteries.
The Southern Tier route contains 1989 waypoints, many more than most receivers can hold. 500 - 1000 is typical, and this is a key specification to look for in a mapping or non-mapping unit. Many of the waypoints are placed on bends in the road and soon after turns so that the course pointer will tell the truth about which way to go.
For a non-mapping GPSR, plan to reload the waypoint memory along the way. If you carry a PDA for email access, that could do it. Or you could take a CD with the data and one of the small GPS programs like G7ToWin. Find a computer shop, Internet cafe, library, or fellow cyclist who would let you load the program and transfer the waypoints.
For a mapping GPSR you could delete the course-pointing waypoints, keeping just those at the important turns and points of interest. Modern automatic routing GPSRs will compute your route along the roads between the remaining waypoints. Just be sure to leave enough to guide its little brain on the roads you want, not the nearest Interstate (grin).
As to specific units, I know the Garmin line best. Magellan makes a competitive line, which you should check out as well. The GPS-V is getting old now, and does not have enough memory for the Southern Tier. If I were in your shoes, I would look hard at the Garmin GPSmap 76C with the City Select map product. It will hold street-level detailed maps for the entire route, using 80 MB of its 115 MB map memory, and 1000 waypoints. The unit plus handlebar mount and maps will cost in the neighborhood of $600.
For the smaller budget there are many non-mapping GPSRs. I would browse the Garmin and Magellan sites, and also look at the extensive collection of reviews, comparisons, and tutorial articles at http://gpsinformation.net