I'd go for the Z. It's just a little more mature. If you know what I mean. There are just too many kids in the scoobies. The Z has a pretty quick 0-60... but the quarter is a bit slow... no slouch but slow for a car that does 0-60 in 5.4s. That leads me to believe the upper gears are long, perfect for turbocharging. I think Nissan may have had that in mind. It's the only explanation I can think of. The Z's powerplant should be more than capable of launching the car in the high 13s in the quarter.
I believe a independant company here in the states has a working turbo kit for it already. Granted it'd be more expensive to mod than a STi, but being a 6 cylinder mill, it has a lot more potential. Just like the 300zx and the Mistu 3000GT VR4 which are good affordable cars to mod by the way. With a 20K budget you can have a 600 WHP car. Couple grand more will get you in the 800 WHP range. Drooling yet?
Speaking of Mitsu, if I was in the market for a STi, I'd consider the EVO instead. Based on what I have seen shops do with theirs, it seems to be underrated at the factory and take on mods much more readily than the scoobies.
If things work out I'm thinking of building a 1st gen Eclipse/Talon AWD. 18k when it's all said and done inside out. 500 WHP. 10s-11s quarter mile and completely street legal.