You dont know what has gone into the rebuilt unit in terms of quality control and parts. Cardone is the largest rebuilder and their stuff is absolute crap. I went through three brake boosters on my DD before I got a good one and had various motors that were basically dipped in black paint and pronounced as "rebuilt."
If you get a kit from Nissan and do it yourself at least you know exactly what's gone into it. btw, brakes are important ;-)