335i owners speak now!!

You're looking at chalk and cheese really aren't you. Both are fast and both are German but other than that it's a personality thing. S3 = Hot hatch, 335i = Sports coupe. I've driven both and know people who own both. Reliability is pretty much the same, both are very well built. Servicing, i'll stick my neck out and say again probably very similar.
Performance wise and as a 'drivers car' the 335i will probably reward you more so. More power and rear wheel drive. The 335i is quicker than the old M3 in a straight line. Depends if you want 4WD or not really.
The beamer will depreciate faster also.

So all i'd say is test drive them both and go with whichever suits you better.

For me the problem with owning a 335i would be the M3. Ive not driven the 335i but have thought about it as a future purchase based on the views and reviews in mags and the guys on here that have driven and owned a 335.
Ive no doubt it is a fantastic car, however for me, the knowledge that they was faster more bonkers version availble would leave me wanting a little bit.

Anyway, I agree they are very different cars but can see why S3 owners would consider the 335 as the `next` car as very few seem to get on with the looks of the 1 series which IMO would be a more like for like comparison.

cheers
Paul