Wifi is faster for me, but I've noticed that my S2's browser is considerably faster (using 3G or EVDO) than my S1's browser was at loading most pages...so it's win/win as far as I'm concerned.
I haven't had been able to get Opera Mini Beta 2 to load pages when in a WiFi hot spot so I've left WiFi off for now. Opera in 3G appears much faster than the BB Browser on WiFi...