Best bang for the buck is to put turbo parts on the NA motor. Followed by putting a turbo motor in (with turbo parts attached...lol). Which is followed by a small block Chevy V8. The RB is pretty far down that list. The VG30DE(TT) might be a bit cheaper than the RB, but only because of availability (of a VG30DE at least) compared o the RB and RB parts.
Why someone would want to put a VG30DE(T, TT) into a Z31 is beyond me, they must have never worked on a Z32, or any VG30DE-equipped car, in their entire life. Now the RB is actually pretty damned easy to work on, and you'd have lots of space on each side for maintenance. Putting it in almost certainly requires relocating the radiator for clearance though.
The RB does have the wow factor, and besides being much more stout than the VG30DE, sounds 31576789249446.356 times better than the VG.
RB swap:
http://z31performance.com/foru...art=0
Chances are that you would be more than happy with just a turbo'd VG30E though.