really it depends on the terrain and how fast you want to go. I have both, and enjoy that option.
4 wheelers do much better in sand and loose dirt, and deep water and mud. 2 wheelers can navigate better through rocky or narrow sections and can go faster over most terrains, and hillclimb better. They are different animals and are ridden differently, but both are fun. A 4 wheeler is obvioulsly more stable because its harder to fall off or loose balance, but also are heavier and would hurt you more if it rolled over on you. And harder to get away from should you need to. and sometimes you do. I think a 4x4 quad is better to have than a 2 wheel drive one, you can sure go a lot more places more safely. And reverse is worth having as well. it depends on if racing it is your main goal or not. To just go riding around, or camping on etc, 4x4 is better.