b2bcrappie -
I have both types that I use for ice fishing. Both Polaris 550's, one a single and the other a two-up. Bought the two-up first to used for ice fishing and an occasional trail ride. The following year, the wife demanded one of her own, so I got the single.
I have used both ice fishing. I have put a hitch on both which helps towing tag sleds. I put a basket on the two-up from dealer, and the added an extra rack on top of that. Both tracks are studded which helps on the frozen snow and ice.
The two-up is primarily a trail sled, and so does not do very well in deep, ungroomed powder. It is bigger, and heavier than the single. It is harder to handle when fully loaded, but it carries a lot of gear, and pulls heavy loads if needed.
I've seen a lot of folks build some custom racks for their singles, but I have concentrated on reducing what I carry so that it fits in one jetsled xl or Shappell Rover, and then either sled works just fine.
The best advantage of having a two-up is being able to easily carry an extra fisherperson who can then help you get unstuck, or help you fish!
MD