Yeah, what jason said.....
Both are awesome powerplants, both have the cool 80's look to them, and they both have great potential.
If I had to choose I would go with a VG in a Z...as personal preference, and that alone. when I had my old junkyard VG on 20psi it planted down 450rwhp with an $800.00 turbo, $500 JWT ecu, $300 intercooler setup and a $400 exhaust, and a few $$ for misc items and some man hours, you are looking at ~$2500 for a screaming 500hp monster and it sounded like a freaking Jet...seriously, it did
pretty good deal to me considering most complete turbo upgrade kits cost more than that and make less power.
and on a plus side, the z31/VG community are coming up with some pretty trick stuff that won't break your budget either, like the Romulator, Poly engine/trans/diff mount, custom aluminum transmission crossmember, and all sorts of other cool gadgets to set your car apart from the others.
If you go with the 7m, do the head gasket at the minimum, upgrade the compressor side of the CT26 turbo, and do some fueling and I'm sure you can do the same thing the VG can. and most importantly...have fun!