I think either will work. The B8s seems to be made with higher performance in mind though. It also seems to be a little shorter so if height is a high priority you might want to use the B6.
Not really sure, but from the feedback it seems the B8s have better damping too. I have looked at some other postings on forums for other platforms too (BMW, Honda, VW, etc.) and it seems B8s are the way to go, but I suppose the only way to get real feedback for our cars would be to find someone who has used both. I was really happy when it was brought up that the ST is already lowered with oem springs because from what I had read, the B8s was what I really wanted but I was beginning to feel that maybe dropping my car might not be a good idea long term. Too many close clearances around here.
That said, the biggest actual difference I can see otherwise that might mean anything to making that choice right off the bat is that the B6s should sit a little bit higher than the B8s. I'm still going for the B8s but I'll likely stick with oem springs.
I wonder though if my oem springs might be sagging a bit...I may try the Eibachs out with the B8s anyway just to see...