The question requires knowledge of how you offroad. Please specify what type of terrain, and most importantly, do you mean with both vehicles stock, or with some measure of modifications?

I will tell you a few general things that are relevant to offroading:
1) The WJ is a solid front axle, while the XK is independent. You can fill volumes with pros/cons, but the rarely contested ones are that solid axles accept more offroad abuse, while IFS drives smoother and straighter (the caveats to both those statements can fill books).
2) In stock form, the electronics package of the XK is likely to do less slipping than the WJ. I'm very impressed with QD2, which is an option for the XK (I have it on my WK).
3) The XK is probably going to have a lot more power, albeit with more weight to carry. In some offroad situations, horsepower has no substitute. In others, lighter weight is very important.
4) I think you'll find the XK is much more top heavy than the WJ. I don't have any data to back up my theory, but having seen both in off-camber situations, the commander looked like a few more pounds on the roof would have rolled it, while the WJ wasn't even considering it.
5) Modifications for the WJ are prolific and inexpensive. Modifications for the XK are the opposite. You will find there are many offroad accessories that simply aren't offered for a WK or a XK.
6) The WJ has a frame, and clearly doesn't need the doors and hatch to keep the structure sound. Lots of chopping options there. The XK, I'm not sure of, but I'm assuming it's designed like the WK, which integrates the body and frame to save weight and add stiffness.

Better offroading? Well, if you want to drive smoothly over rough roads at higher speeds, the XK should do better. If you want to wedge 37" tires under it and go rock crawling, the WJ gets my vote. Neither vote has much certainty tied to it.

 

6) The WJ has a frame, and clearly doesn't need the doors and hatch to keep the structure sound. Lots of chopping options there. The XK, I'm not sure of, but I'm assuming it's designed like the WK, which integrates the body and frame to save weight and add stiffness.

The WJ does not have a full frame , it's a unibody just like the WK/XK and the ZJ before it .

 

I didn't know that; thank you for the correction. I don't know why I thought I'd seen a frame on one. I may have confused it with a land cruiser or something else. My apologies.

I will stand by my observation that WJs do not need the doors and hatches, because I've seen so many offroading without them. It seems to be a thing. I haven't seen a WK or XK stripped down like that, so I don't know if they can't handle it or if just nobody has.
I can say I've observed my rear hatch won't open on certain terrain, and I've been warned that if you open it while you're stuck, you might not be able to close it again until you get on better ground. I totally believe that. For all I know, the WJ does the same, I just don't know.

 

Okay guys this is how I offroad. I work in farm/ranch country (western great plains) and I'll drive on highways for 50 or 100 miles and than off road for the rest of the day accessing job sites (construction). Some sites are pretty remote and rugged (mesas and buttes and the like). But I like to keep the rigs close to stock as much as possible because of the highway driving and that's why I like jeeps, they off road great at stock level. My favorite is the XJ. By far the best for offroading but not the most comfortable on the highway. The WJ does both well.
I meant to say I got a bead on an XK, that's why I asked. Thanks guys.

 

Given those parameters, I think you'd be much happier in the commander, but I would also suggest you look at the WK. I drove both before buying the WK, and the commander was noticeably more sluggish and wallowed in corners. Not that I didn't like it, but the WK was noticeably more spry.

These QD2 rigs are freakishly good offroad, bone stock, and drive very nice.

 

I went from an XJ to my QDII WK. Thus far the WK does everything better than the XJ, aside from handling corners and situations where the extra weight is a hindrance. 99% of the time though I much prefer being in my WK.