256mm brake upgrade 48mm or 57mm piston calipers ?

Hi all,
I am the new owner of a 1990 cabbie gti. It had an unfinished 16v conversion and hasn't been on the road since 2008. It's in very good nick bodily and now runs,  but the problems of 8 years unused are evident. The engine is now running, but the coolant is coming out faster than I can poor it in  - new metal pipe on order.

Whilst waiting for parts I have been doing the brakes, I've installed the 9" servo, a remote reservoir and a 22mm master cylinder. I like the wheels I have, so was thinking of upgrading the front discs to 256mm. The caliper choice is interesting though, I have found astra/corsa units with carriers which will fit, they are 48mm pistons and take 20mm max thickness discs  - mk2 gti would fit. Alternatively, I have found vectra/saab 93 units wit carriers which fit which are 57mm pistons and take 25mm max thickness discs  - 24mm cavelier discs would probably do.

I am now stuck as I don't know which way to go and which will give the best results. I guess the 48mm is same as stock, so small pedal travel will give improved braking from the larger discs, the 57mm I guess will need more pedal travel,  but what other advantages?

Advice appreciated!

Thanks

2nd question, are the carriers for the vectrs and Corsa the same? I.e. will they both accommodate  256mm disc?