I currently use a GPS V from Garmin on my snowmobile. It has been going several years at temps down to -15 with no problems. Its waterproof, can be put in a pocket and doesn't need to be hooked up to the sled to work. It will run all day on a set of batteries with the backlight off. I run sled power to it with the backlight on max brightness to keed the LCD warmer. It aids the LCD display from being so sluggish. This gps also has autorouting built in. The only trick with this or other GPS units that use AAA batteries is to put a strip of streched electrical tape across each pair of batteries from one end to the other. This prevents the vibration from messing up the battery terminals.
As a bonus, this is a long discontinued unit, so it should be cheap on ebay. Its nice to be able to locate gas or food while on the trail. While the 26XX series is real nice, its real heavy the have hanging on a ram mounts while slamming through the woods. Besides, if somebody stole my GPS V, I could deal with it. Losing my 2610 would hurt.
The only downfall of the GPS V is that it has 19 megs of memory and uses a serial connection to the computer. It takes about 45 min to upload 1/2 of Vermont into the GPS. That does include every road, gas station, hotel, hospital, food, ect so thats all good. I find that having 1/2 the state at a time is more than enough. If you dont need all the added data, dont upload the street data, just use it to keep you from getting lost.
No matter what you use, get a unit that is designed as a handheld. The displays are smaller, but if your sled brakes down and you have to walk, you want a unit that can run a long time on batteries. Handheld units also have many more features than the car style units. Ram Mount make mounts that will allow you to mount almost any GPS to your sled.