My two cents... if the ZJ has the 5.2, go for it. If you really want an honest side-by-side on the mpg's between the two under actual driving conditions, the 4.0 wins by only about 1 mpg, IN THE ZJ; the XJ will do a bit better, being lighter. And the 249 is problematic, but most guys do the 242 swap when the VC goes bad. Very easy swap.
If they've both got the 4.0, go with the XJ. Just remember the rear leafs' history of going flat...
You don't say where you'll be living (at least I didnt' see it), but if you'll be in the rust-belt, go with the ZJ. The XJ's don't GENERALLY fare as well with rust.
They both use the d30 up front, but the ZJ's - like the 00-01 XJ's - are LP. Lots of HP swaps are done, though. The ZJ rear will have either a d35 or a d44a (offered with towing pkg through mid-95, after that was standard behind with the V8's), which while is weaker due to the aluminum diff housing, has even heftier guts than the steel 44, and with a decent truss is said to exceed the strength of a d60. Drawback: VERY limited locker availability; in fact, I think production for 44a lockers is a thing of the past. The 8.8 swap into a ZJ is a bit easier than with the XJ; this is not based on personal experience but on what those that have done it to both have stated on various forums I've seen; ymmv.
The AW4 was used in early 93 models. The 42re behind the 4.0's does have a history of going bad, but it's almost ALWAYS an issue with lack of maintenance, specifically NOT checking cooler lines and cooler tank for clogging. Also, the 4X series (42re's, 44rh behind the 93-95 5.2, the 44re behind the 96-98 5.2, and the 46re behind the 5.9 - only available in 98) need band adjustments. The "e" is for electronic, the "h" is for hydraulic. Well, more to it than that, but....
It has always puzzled me why we don't go to other forums for information. Instead, we tend to get stuck in "us vs. them" mindsets. Not just here, not just with Jeeps, but everything I know of - primitive archery vs. compounds... gun vs. bow... even calibers of guns!!! Tires. You name it. I don't know, I say get on over to the ZJ forum and look around. You guys on here could open a whole new WORLD of "DEATHWOBBLE" info for them; if they came over here (objectively, lol) the debate between the 4.slow, as it's sometimes referred to, and the 5.2/5.9 would have more meat to it.
I've had just about every kind of Jeep - just never owned a Scrambler/CJ-8, Wranglers (YJ, OR TJ), the Liberty, WJ's (99-04 Grand Cherokees), Commander, or the poser Jeeps. I've had the FS Wagoneers before they WERE "Grands," the FS Cherokees, pickups (including an MJ), XJ's (I just gave my last XJ away to a youngster about 2 hours south so he could build it for a project), and now 2 ZJ's, both with the 4.0's, 42re's, and 242's. I've had (didn't own, they were on the farm, at our camps or whatever) FC-150 & 170 as well, plus other stuff I was too young to know that there was a difference. It's been Jeeps. The real ones, lol! In a very real sense, "It's a Jeep Thing" doesn't mean ONLY "YOUR" Jeep... or only one model.
Rant off.. next!