It is a hard pick between the Z and STi. One is nicer inside, smaller, RWD/AWD...
Aftermarket wise, I strongly doubt that the VQ will have a less aftermarket support compared to the US-sepc STi motor.
The Greddy turbo kit is not finalized yet, and I won't be suprised to see 350HP, 375HP is pretty likely as well. Plus, VQ engien is used on a lot different models.
The STi, on the other hand is US only, how many US-spec STi will be out there?
So, I think it's a tie. As far as the real potentional of the engine, we don't know on both of them.
If you wanna get the Z, then go with the track model, and you get the Brembo brakes, then you will need extra $$$ for the turbo kit, which I would rather just get an M3/Vette in that case.
In another word, if you more performance then what the Z can offer, don't buy it and get a turbo kit for it. Not really worth it, unless you have special, real special feelings for the car.
If you will be happy with what the STi can offer, performance, interior, space, value wise. then go with the STi. But to be very honest, I doubt that there will be a very strong aftermaket support for the 2.5. At least not when the car first came out. And when they do have some stuff, most likely that it's gonna be expensive, and won't be worth it, cuz void your warranty as well.
And honest, with 300hp, you sure that's not enough for you?