I have the 4 banger and I run 31" tires just fine. I do have some problems with my engine (I think I got some rust on the tank, and since the previous owner NEVER changed the fuel filter, I think my fuel injectors are done for, butI will be working on that problem this week; I also need to clean out the TB and intake manifold, as it is filled with dirt, and I will take the opportunity to replace the 2.5L TB with the 4.0L one, which I already have), but even with these, it turns my wheels just fine.
When you hit 65mph, it is like hitting a brick wall, so it's tough going past it. And the fuel consuption is defenetly not the best (after all, a Jeep DOES have the aerodynamic characteristics of a BRICK!), but it is a lot of fun to drive around. I actually like the fact that there is a lot of room in the engine compartment, which makes it easy to maintain. And I also like the cheaper price tag!
If I would have to do it all over again, I may opt for the 4.0L, but I do not regret my choice at all. So if you have a choice, and costs are not so significantly different as to make it beyond your budget, I would go for the 6 cylinder. But if not, I would defenetly get the 4 banger.
Just read up on all the mods you can do to make it a bit more powerful. There is no one mod that will give you a lot of power, but a lot of little mods might make it worth the time and money. Hey, after all, there are some folks out there runing 33" tires on the 4 banger with no gear changes, and they say they dont have any problems (except for not using 5th gear)...
Just my 2 cents.
Felipe