I would personally go for the Z32, they are just a sexy looking vehicle. They are extremely responsive to modification, and with a 3l donk have torque like you wont find in the RB20 even with major mods.
The VG30 is an extremely strong engine, however most are now over 15 years old so look out for signs of trouble. Basically the bigger the pile of receipts the current owner can provide the more you should take that car