I would get the Xterra over the r50 (of the year range you specified) just because of the motor. The 3.3 (just like the 3.0 in the old pathys) is a proven motor and I would trust my high mileage VG motor to take me into the middle of nowhere over a VQ just because it has been proven a workhorse.
The r50 steering is actually pretty strong I've never hear of anyone ruining the steering rack, at least on this board. The only weak part IMO is the unibody. But I have heard that adding skids adds to the rigidity
If you do get an xterra first things I would do is beef up the steering(aftermarket CL, heavy duty tie rod adjusters), get some skid plates and some good tires.
They are both great trucks but I lean more towards the xterra because of the motor and body of frame. Also don't get a supercharged one as I have heard they are problematic.