Until 3 or 4 years ago the Pathfinder had the same 3.3 VG motor that the Xterra had. Then they switched to a 3.5 VQ, which is quite a bit more powerful and supposedly more refined. Pathfinders have nicer interiors (both in materials quality and features), but they cost a bit more. You can check out carpoint for all the mechanical differences (unibody v. body on frame, coil v. leaf, VG v. VQ, etc).

Personally I prefer the Xterra - a bit more durable (because of the frame) and better style. And with a 5-spd I don't think the power on the non-SC V6s is bad at all. On the downside, they don't ride/handle as well, they are a bit slower, and the interior/features aren't as nice.

Pathfinders are nice too, so if you get a good deal I'd jump on it.