Was on late last night and was tired so didn't give much of a reason why hah.
Basically i'd go catless because you'll have lower EGTs, less backpressure and less weight (albeit like 8 lbs, but whatever.. it all ads up). Because of this you will be able to spool quicker (more flow = faster spool), and eventually be able to tune better (lower EGTs and such).
I've heard people say that a bigger downpipe (like 3in up from 2.5in) will cause an increase in boost, but that literally makes no sense to me as a stock wastegate is pressure controlled by the spring anyways.
To sum it up: from the turbo back, bigger and less impedance is better