Got to be petrol.
I had a 335i and after being loaned a 330i for two days I sold the 335i.
335i was mapped and seriously quick, quicker than my CSL, but all in all it is a turbo so can get a bit boring, power is always there and you never have to work it.
I wouldn't say that either are that fun though to be honest, if you have a 5 series as well get a nice 130i M-Sport or a 135i coupe..
Somehow the 35i engine seems to work in that go kart.