it all depends on where you want the power, my zetec turbo reved more freely at high revs with a .48 t3, and i would say the boost came in about 200rpm later, no difference in actual lag time as the turbos internals are the same size. I had a .63 t34 on my 2litre zvh, it made full boost at 4k, was really laggy as the turbos internals were a lot heavier. Too big IMO for the power i was making