If it's strictly for icefishing, I'd pick a snowmobile.. If you are going to pick a 4wheeler, definitely 4wd.. I've used both out on the ice, the only time the 4wheeler really struggles is when there's a lot of snow or a lot of slush.. They tend to sink compared to a snowmobile in the same conditions.
When you get into back lake icefishing, snowmobile hands down.