how handy are you with spanners or how much cash do you have in the bank?
I went 1.8t as it is emminently tunable for the size, you only need 4 of everything for engine components and one of everything turbo/mani/exhaust wise and the only thing that requires the engine being pulled is changing the clutch.
a set of rods, beefier clutch and a big turbo conversion and you could be well over 400bhp at a reasonable cost.
add some uprated valvetrain and a few bits and bobs and there are GT35/40 equipped 1.8ts running nutty power
by contrast the S4 has an awesome sound track will get you potentially higher gains and a smoother power curve but at a cost.
6x everything for engine upgrade
double the amount of turbos, manis, CAT/decat, exhausts and you better get very comfortable pulling engines if you dont have a ready supply of cash for someone else to do it.
400+bhp on an S4 is potentially easier to reach...but IMO will cost you more
if you only are looking for 300+bhp...just get the S4 and be done!