2003-2006 is best. Limited has nice options.
Unibody is more like body on subframe than true unibody. No body flex or noises. Extremely solid. No comparison to Xterra. Xterra is a toy in comparision. Front differentials are a joke (very weak, aluminum housing). The Montero uses 9.5" ring gear in the rear (similar to 1/2 ton trucks); Tacoma is only 8" in comparison. Front is 8.5" ring gear; Toyota uses 7.5" in FJ, Taco, 4runner. It is massively overbuilt. One of the best transfercase / 4wd systems of any SUV built. Best traction control system I've ever seen (you need to experience it).
Montero looks like a soccer mobile. Xterra looks tough. But in reality, Montero will dominate in build quality, strength and capability.