I love both Stefan ("STEF-an") and Stephen ("STEVE-n"). I don't like the Steven spelling. Between the two, it would depend on the surname. Stefan would go better with a Scandinavian or German last name than Stephen. But either one would be great.
It's okay, but I always think it's just somebody who wants to be trendy with Steven - like Steve Urkel! I definately prefer Stephen, but Steven's okay too.