I've been using GPS for close to 20 years both on land at at sea. I have even taught a few seminars to neophyte navigaors. I am not saying all of this to blow my horn, only to illustrate that I have dabbled with these things a lot over the years and so have a bit of experience on what works and what does not.
My advice is to go with a Garmin and in particular the 76S or 76CS or CSX. Garmin is a proven model, works well under even marginal conditions and best of all....really really best of all, it is user-friendly. Failing that, the Garmin 12XL is an excellent compact model, although I am not certain if they are still being produced.
As already mentioned by Ray, carry an extra set or two of batteries. Suggest lithium batteries as they last significantly longer and stand up to the cold far better than ordinary batteries. On that note, cold weather will kill your batteries very quickly, especially with the power-consumptive GPS.
While you are at it, purchase the computer program "Map Source" which is great for manipulating data on your GPS via your computer.
Finally, I concur with Ray. Carry a compass and a map. The one thing about electronics is that when they fail, they fail absolutely.
my two bits' worth.
Robert