I have to say it all depends on how much money you want to spend and what your power goals are. If your goal is a moderate 300hp to the wheels I would have to say the cheapest way to get there in my own opinion is by using the EJ20 WRX engine. I got a ride in a WRX that dynoed at 278 to the wheels. He only spent $5000 in mods and labor to get there. However the downfall is he spent more to buy his car. So a $7000-$9000 Turbo setup/Exhaust/Engine management/Clutch-Flywheel doesn't make the total cost of the car come out more expensive then what Reggie paid for his modded WRX.
So in a twisted way it is up to personal preference. Would you like your turbo with round headlights or Classic GC8 style?
Now in the long run when it comes to peak power potential with a complet disregard for money I feel that both engines would come out fairly equel. You may say the EJ25 has more displacement. Well you could always bore and stroke the EJ20. Both engines cylinder head will need stiffer springs new valves and a good port and polish. Both engines could benefit from an intake manifold redesign. Both engines definitely need forged pistons and rods.