If you ignore the fact I have bought a 35iS and it has broken down (Grrr) I have driven both the 35i and bought 35iS. IMO the 35iS is far better. I tred a newer 35i and hated it, mainly due to the suspension. It was £32K, newer than the 35iS and a third of the miles but was not for me. It just didn't 'grab' me like the 35iS did ! Why was that ? Well ...
The suspension was seriously hard on the 35i and not 'adjustable' whereas the 35iS with the variable suspension was much more compliant. Normal is great. Sport is superb for pushing on and altering the steering / gear change speed as well.
Steering - just seems better weighted in the 35iS as opposed to the 35i.
Exhaust - I did not really notice the 35i exhaust whereas the 35iS was great - not massively louder but far more noticeable (you would have to try to see what I mean). I love the 35iS exhaust at +10db. I was worried by a review that said it was boomy at 2500-3000rpm. What utter rubbish. It is an absolute ideal mix of sporty but not aftermarket boom. There is not boom at all - just a quality that is missing with the 35i.
Gear change - I am an auto person. I have driven BMW autos as a policeman in 530d's for years and am very used to autos now. I recently tried a manual in a Fiesta ST after my 330d and though is was actually superb I missed 2 pedals ! The DCT is great. Ultra smooth changing up and seamless. The only complain compared to a 'normal' torque converter auto is the downchange can take a while whilst kicking down (due to it being a twin clutch box). This does just give you a chance to hold onto the steering wheel a little firmer for the surge when it comes
Acceleration - not massively different but one feels really fast as opposed to just 'fast'. 35bhp does notice more to the top of the rev range I would suggest.
Try both and buy a 35iS :wink:
Just buy one better than mine which after a week and 500 miles developed a misfire (something I think all of the 35i engines are potentially prone to!).