Hey, I've owned a few ATV's in the past, and used them for back country fishing as you will.
1. Polaris Magnum 350 - Underpowered but a decent machine. Unfortunately this was stolen after I owned it a few months.
2. I replaced with a Magnum 500 - Lots of power! Very capable and I owned it for 2 years with no issues.
3. I traded in the Magnum for a Yamaha Rhino 420cc because of all the gear I carry, and wanted to take passengers with me. I absolutly loved the machine and owned it for several years - Did much ice fishing, backwoods fishing and trail riding with it.
As was said its your money, get your opinions sit on a few and test a few if you can, pick the one thats right...