One major difference: the 35is has a sportier exhaust system that sounds better! (Note Z4 35i dual exhaust systems are very expensive being up to $2500 to improve the sound).
It also comes with DCT as standard and the sports suspension.
Also if you want to be particular it has grey instrument dials with 35is written on them.
It is said that the BMW has slightly underrated the 35is power. I have seen a dyno somewhere. I was suprised that it 'only' produces 340 odd HP given that it runs around 14psi on overboost.