Similar performance and both very tunalble, but very different cars so hard to choose on performance alone. J-spec Scooby specs vary quiet a lot dependent on version which can be WRX, WRX STi and a number of different model year versions, genrally fom what I know, J-Spec are more powerful than the UK versions but I believe some of that is down to ECU mapping for running on higher octane Jap fuel, you may lose some of this advantage if you get it remapped for lower octane fuel. My choice was based on buying for my wife, she drove the classic (pre 2000) Scooby which she did not get on with, then the GT-4 which she loved, so the GT-4 was the one (I am getting a new Impreza WRX soon though!). You will find the Scooby is more to insure if that is a consideration, but really you should try them both and see what suits you, both FAB motors! One other thing, you will find that Toyota support grey imports in the UK and will service and supply parts (at fair prices), I belive Subaru do not support imports, so you will using specialists (lots of them about) rather than the main dealers.
Oh yes, I also meant to say on my earlier post that many GT-4s have the GT Four raising blocks added to the spoliers by their owners so it is not a reliable way to see if its a real WRC version, there are some ways of being sure, detailed via a link on the GT4OC web site.