Wow, one WD21 owner who didn't slam the R50 pathfinder!?
Hi Yukon, Welcome to NPORA.
There are a lot of differences between the R50 (1996-2004) Pathfinder and the Xterra, as some have pointed out above, but both are very good vehicles. We have a 99.5 Pathfinder, and a 2000 Xterra in our driveway.
I think most would say the Xterra is better offroad, they are certainly the more popular choice, and there is quite a bit more aftermarket support for protection & upgrades. That said, I have wheeled my pathfinder with the Xterra club for 5 years now, and I am able to keep up, and in some cases piss people off because I got through in my lowrider pathy.
You can easily lift an Xterra with body and suspension lifts to fit 33" tires. The pathfinder has a 2" lift kit available, which typically allows 32" tires. (of course, there are exception to the rules, and then there are custom kits that can give either truck more lift).
Lifted Xterra's develop steering issues (as the older WD21 pathfinders, they wear), and you will need to upgrade & maintain steering components.
As I mentioned the Xterra does have better aftermarket support for things like skidplates, sliders, bumpers,snorkels as well as lifts and other toys. That said, if you do your homework, you can also outfit the R50 pathfinder.
Size wise, the R50 is larger - a bit longer & wider, which gives it more in cabin space. Although, I have slept in the back of either truck comfortably.
As mentioned the Xterra is a body on frame, which gives it a more "truck like ride", where the caR50 is a unibody, which is a nicer ride on road.
I would suggest you test drive both, and compare for yourself. Good luck with your search.