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I would go with ATV, as it's a 4 season machine.
Yep
Also depends on how much money you have to spend.You can get a decent used sled far cheaper than a decent used ATV.
Lighter the better man. The lighter it is, the earlier u can put it on the ice.
You didn't mention if you need to trailer/transport the Quad or ATV to the lakes you fish? Having to deal with a trailer can factor in here as well. I own both and use both for ice fishing in different situations. For me I tend to use the ATV a little more because of the trailer situation, takes me a lot less time to run it up in my utility trailer and throw some fishing gear in with it and take off. With my sled I need to put it on my two place snowmobile trailer and it is always hard to strap down things like a flip over etc (always afraid they will fly off on the road), so I end up packing everything in the back of my truck. With the ATV and utility trailer I can put a bunch of items in it with it and keep the truck pretty free. Again if only 1 I use my ATV year round even for small things around the house. But if deep snow or glare ice, the sled is nice to have.