In my opinion the R50 ('96-'04) Pathfinder and 2nd Gen (N50) Xterra ('05-Present) are pretty comparable in interior space ... I've got an '08 Xterra Offroad and my wife drives an '03 Pathfinder. The X sits a bit taller, but I've also got true 33s ... because of where the interior seating is, in relation to the rear axle, the Xterra's rear seat can be a bit tough to get in and out of (getting over the rear tire 'hump'), but only if you're more limited in mobility or can't open the rear door all the way.
I wouldn't want to go three across the back seat in either one, even if it were three kiddos. The cargo area in the X is a bit taller than the Pathy, but I bet you could cram the same stuff into either ... we did two couples for camping at a weekend triathlon event (with way more 'stuff' than we needed), in the Pathy, and it worked fine to and from. I can sleep my 6'4" self in the back of the Xterra (rear seats down), but don't think I could do that in the Pathfinder.
The factory roof rack on either is actually pretty darn good, unless you're talking about more than a couple hundred pounds of gear; then I'd look at a replacement rack ... I know of many options for the X (and have a Gobi myself), haven't looked into it for the Pathy. The factory locker in the m226 axle (Xterra rear, D44 variant, available in Offroad trims and with manual transmissions) was upgraded from two to four spider gears at some point (IIRC it was in '07, but don't hold me to it). ARB has lockers for all rear ends in either vehicle, at this point ... talk to the folks at Rugged Rocks Offroad, they'll get you squared away if that's the direction you decide to go.
In the R50 Pathfinders, look at the later model years and see if you can get on that had a VQ35 in it ... IMO that's a much better powerplant than the VG33 (which is really not much of a slouch, in it's own right, just underpowered for the R50). Both of the VQ motors (the 35 in the Pathy and the 40 in the X) are virtually bombproof and about as good as it gets from a balanced power & efficiency standpoint ... for example, I still get 16-17 mpg around town and upwards of 18 or 19 on the highway; and that's with full underbody skidplates, rock sliders, replacement rack & lights, and heavy 33" Toyo MTs.
In the X, look for a manual trans if you can ... I know it's not everyone's cup of tea, but if you enjoy driving manuals you'll be happy with this one. It's a smooth trans, and I think the benefits of not having to worry about the SMOD, and being able to stab the clutch and kill power to save the stock front diff, are very real ... and far outweigh any possible headache of needing to tapdance offroad (I avoid traffic at all costs, and don't really have much rush hour to speak of - so that may change the equation if it's a concern). Plus in 1st gear 4-Lo, the crawl ratio is such that you can start the truck without clutch work and not need to worry about any lurch ... it takes a time or two of practice, but I could probably almost get it to start up and idle over a curb, at this point (maybe that's a good test, I haven't actually tried it yet ... but now I kind of want to ... lol).
Any more questions or specific comparisons, feel free to ask away ... I'm on NewX as well. It's a good group over there, and there's really good info to be had.
