I've never heard either exhaust in person, but I would go Gintani. I say this because it seems that everyone likes theirs, and I still remember the whole Eisenhaus scam with the Eisenmann Meisterschaft series of exhausts in the E9X 335i community when I was looking at getting a E92 335xi. If you don't know what I'm talking about, I'm sure you can find plenty of threads, but basically people were ordering Eisenmann exhausts and some people got them and some people got what they now call Meisterschaft exhausts.
"The laws of physics cannot be repealed, even with DSC." - BMW Owner's Manual