In an all motor application, both the B16A and the B18C are equally powerful. The B18C has some more torque out of the box, but the B16A can easily be made to match.
If price is an issue, the B16A has less initial cost, and there are more of them around than the B18C. That's not to say it's better, but a point nonetheless.
Both SiR and GSR trannies are great, the B16A tranny has shorter gears and would be a better setup for a drag application. The GSR would make a great tranny for boost. (Keep in mind I am just generalizing here)
In my honest opinion, If I was just starting out again I'd go for an SiR Longblock with a JDM B18C Spec R Transmission. The spec R tranny has the best final drive out of the B-series trannies, plus dual-synchromesh for added durability (not to mention a pretty kick ass Limited Slip Diff). Use the Spec R Clutch assembly and flywheel if you can (unless you want to go crazy now with a clutch....I have them on my SiR and it's a real plus)
If this sort of freaks you out (mismatching trannies and engines) don't worry, it's a direct bolt on and would be no diffrent than putting a B16A together. And it is totally worth it in the end.
As for the F20, I personally wouldn't bother. I'm sure it's a capeable engine, but there are better choices out there at similar pricepoints.
Good luck and enjoy the sunshine you lucky bastard (1 Foot of snow this weekend for me!!)