the biggest pro of running something like a 15" wheel with 35" tire is that they are cheap and available just about anywhere. aired down, you can get a nice wide footprint and have a lot of rubber to work with. the con of this is that more sidewall means more sidewall flex and in off camber situations, this isn't a good thing. another con is that the brake calipers on a JK are big enough that some 15" wheels will not fit without making contact. griding can be done to make this work but, i personally would prefer not to do that. 17" wheels and tires for it are pretty common and can be found for them easily as well and at a reasonable price. they offer less sidewall flex and you would have no clearance issues with them. 16" and 18" wheels offer a limited selection of tire choices and i typically try to avoid them. if you run 40's or bigger, 20's are nice to have but, you will find more tire options with 17" wheels.