i'm a zj owner, i wouldnt necessarily say they're better. my roommate has an xj, he's got a 3 inch lift, i've got a 2 inch lift and we both have the same exact 31" tires. most of the things he can make it through, i can make it through as well, however, he does have better ground clearance, so that helps him in areas. (he's also got a winch) but another thing i noticed is that his is minimally smaller, but in tight trails, it becomes very noticeable.
i say, to each his own, xj's probably have more parts out there for customization. but each can be just as capable as the other if they're built right. and even bigger, driven right, the driver is infinately important.