B6s are pretty expensive, and then there's Rabbit hole of MK6 vs MK5 B6s to go down as well, which is why I just got MK5 GTI spec B4s for an easier life
B4s are pretty good as an OEM like-for-like replacement, but I don't think they like stiff springs or uprated ARBs though. Bilstein say the B4 can be used with mild lowering springs, but just depends how much stiffer/lower they are compared to stock.
B4s + Stock springs + stock ARBs = No problem. Works as expected.
B4s + H&R or Eibach springs + Stock ARBs = No guarantees, but a lot of folk get on with that setup.
B4s + H&R or Eibach springs + H&R or Eibach ARBs = Horrible. You definitely need B6 or B8 for that combination.