Does size matter? Excuse the pun. S3 is the smaller lighter car but the S4 V6 super charged engine and torsen quattro is hard to pass up though. S4 also has more torque too.
I would suspect the S3 would be quicker as it's lighter, what are we looking at, S3+bolt ons + map = 400/400 or so, S5 pulley etc. = 450/450 ish? I think the S3 would have more BHP/torque per ton.
However one will sound like a 4 pot, and one will sound glorious
I'd buy the S3 if all you want to do is burn up the 1/4 ..
Simple. Wait for the RS3 > S3 > S4