well the RB26 might cost a bit more upfront, but it is lighter (i think) than the VG30 and it also does have the potential to make a bit more power than the VG30 on the standard internals, as well as the fact that you only have to buy two cams if you were to upgrade (saves you a couple hundred bucks), and i think it fits the engine bay a bit easier, and makes things easier to work on.
the VG30 is pretty wide, but could probably sit a lot further back in the bay, allowing it to have a bit better weight distribution.
I Just think the RB26 might be easier to work on, however, a lot of the parts you're going to buy will have to be special order.
Either way, you'd have to use the transmission from the VG30 if you want to have RWD 5-speed. So unless you can find one on it's own, or get an RB25 with the 5speed instead of the AWD (some shops will do this, as they've already taken the hint that most applications for the RB series won't be going into a AWD car or use the standard 5 speed AWD transmission or the 6 speed getrag.
If you'd like to make it economical and better bang for the buck, go RB25DET. can't go wrong with that combo.