Which STi????
As the previous owner of an RB5 Prodrive, I can say with confidence, its probably the best handling Impreza on UK B-roads of all time! You can get more powerfull Impreza's but in real world driving they end up slower than the RB5 because they don't handle as well!
I now drive an MY03 STi which is now a far better car than the RB5, but only after spending a significant amount on handling upgrades - 18" rims + decent tyres, Prodrive springs, stiffer ARB's & drop links.
My advice would depend on what you have to spend! If you want a Classic and don't want to start making handling mods, then go for the RB5 Prodrive. If you can afford a new-age STi and some mods to boot, then that's the one to go for - with extra work the new-age STi can be better than the RB5!