Depends....
If you say your going to use it for ice-fishing only. I would go with a sled. Especially when the fishing is slow
But I doubt you will use it for fishing only. Chances are if you bought a quad you will find plenty of uses for it in the summer.
I would go with a quad, if you have uses for it in the summer as well.
But, if you live in a place where there is lots of snow or a long winter, go for a sled. Quad sucks in deep snow.