This is a fairly old thread, so I don't know if perhaps you have made up your mind already, but I thought I would add a few thoughts...
First, you can rent both of them from Avis. I HIGHLY recommend doing that for a weekend, and then go put a ten hour one way trip under your belt. See what you think in the city and on the HWY.
I have done that, and I like the trucks well enough. I liked everything about the FJ except one thing, Ray Charles could drive the thing with ease! By that I mean it has so many blind spots on that ruddy vehicle that Ray Charles would not be any worse off than the rest of us. Beyond that it is comfortable, and enjoyable, but again REALLY REALLY bad blind spots all over the place. Further you can hardly get a GPS to work in the thing because you have to stick it so far forward on the dash just to get a signal, at least you do in the Mountains. The power output of the engine was, well, it was alright. You certainly will not win any races, but it did not feel like it had any lag, just lug. It was a slow truck; at least the thing did not make a ton of noise and then do nothing, it was a quiet nothing. The last real complaint about the thing is I would NEVER want to have to do a car seat in the back of this thing, nor do I EVER want to be the poor sucker to have to ride in the rear. I climbed in there to see what it was like, and, well, the Mazda 3 I just rented has as much leg room and comfort in the back seat as this thing has. (very little) The thing does have that optional camera in the rear mirror, wow that is a nice touch for sure. (especially as you can't see out of the truck without it.) I really liked that thing, and now I want one in all of my vehicles! Some have said they are not strong off road... I think they do just fine, they are what they are, and for a stock truck you can't get much better. (but you can get better, just not much)
The Unlimited, it was kind of ungainly in my opinion. I drive a WJ, and so I have that to compare it with (and a few dozen rental cars from all of my business travels.) It rides ok, but it is a wrangler, so ok is what you expect. The top down is wonderful, of course, and easy enough to do. I guess I am being overly harsh on the jeep, it really did ride nice enough, but it felt like it needed more power. A nice V8 or peppy torque filled diesel would be great in that thing. The ambient noise level was certainly higher in the Jeep (IMHO) than in the Toy, which makes sense, the jeep has no insulation, even the hard top does little to keep out the noise, it is intended to come off, not stay on.
Both vehicles have a nice shallow rear overhang, which is a very good thing.
Well, anyhow, my thoughts.
Best of luck, rent both and narrow down the odds.
Brian
PS, To Black ZJ up there... oohhh dang! Wow that really sucks. It also has to be fairly hard to do! I have done Kane a few times, it is a fairly easy trail all things considered...