It doesn't make huge differences if the discs are drilled or slotted
or both in you case. You won't go on track I guess.
The biggest difference comes from disc size, calipes (pot and pad
size) and pad type (friction).
Right, for the larger FN3 calipers (with 288 x 25 or 312 x 25 discs)
you'll need new hubs for 55 mm strut. You find find some used
hubs as some GTI owners go for the alloy hubs from the S3/Passat
(as me).
No matter if you stay with the stock calipers (FS III) or upgrade the
2.0 TDI/GTI ones better discs are highly recommended. Ate Power
Disc, AP Xtreme (= Brembo Max), EBC TurboGroove or even Tarox
is a question of your budget. I'd go for street legal Ferodo DS
Performance pads.
The former Golf GT (170 HP) had the same combination with 312 mm
front and 255 mm rear as you might go for.
I don't know the details but there is a newer option for 270 mm discs
at the rear which VW uses for some newer Sciroccos.