Just my experience, hopefully will be helpful.
Some time ago, bought a "mid tier" GPS from Garmin, at the time, got it for a great price in the high $300 range (best price I could find it for at the time, which was at Abe's of Maine).
Long story, but do to my own stupidity (trusting someone), I had to buy another Garmin (yes, I did like using it that much for work, as well as the wife).
This past Christmas, I ended up getting the Nuvi 260W for $189 at Best Buy (think the unit is discontinued, perhaps a reason for attractive price).
Like all technilogical products, prices come down, and features increase.
I actually like the 260 better than my older Garmin, and it was a lot less money.
That said, I personally know of four people including myself who have had to send their GPS unit back to Garmin for a "new" refurished unit because for some reason or another, the unit wouldn't "track" anymore. I had my older unit for 18 months when I had to send it back. Garmin (like most other makes out there) usually come with a standard one year wrty. My refurbished unit cost me $100 plus shipping. Talking with Garmin, my understanding from them is that the newer units have a built in attenna that reduce the chances of the unit failing to "track" over time. My older unit that I had to send back had the "flip up" attenna. I can't help but think that an "outdoor" unit will get more abuse, so depending on what type and cost of extended wrty someone offers, I seriously think about it.
As mentioned, check Garmins website. Needless to say, Waterproof will be a must for outdoor use (as well as battery powered) but my 260 unit will also run for a decent amount on its battery without having to use a car charger (you can buy an optiontional wall charger).
However, I always wonder how waterproof something is unless it's meant for scuba diving.
I'd check online for information on the units that interest you, try to find unbiased reviews of the unit that holds your interest, then see if you can play with one at a electronics store. Check the best prices online as well just to make sure you're not paying too much if you go to a store.
I will note that I really do like the moveable "sand bag" mount (to actually hold the GPS unit) that you put on the dash. I didn't see them out when I got my first unit, but when I got my second unit, found them. This way, you can remove the mount for the unit with no problem and store it out of sight. Personally, leaving the standard windshield mount stuck on the car glass when the car was left unattended for a decent amount of time (like at the airport) was only a greater invitation for someone to break into your car. Needless to say, with an ATV or snowmobile, this will not be an option
I really don't know of anyone who is not really happy with their portable GPS, and I've heard nothing but good things about the Tom Tom (although I don't know if they have an outdoor unit, but would make sense that they would).