Here's my 2 cents - having owned an E90, and driven the F30 a couple of times (both 335i's and sport pkg equipped). And this is IMO, coz' I don't want to be flamed....this being the F30 board..
The E90/92 seems to me to be more of a sport/handling focused car, where as the F30 seems more of a luxury/slightly softer feel oriented car. The biggest difference to me was from the steering feel perspective - the E90 steering is 'alive' and IMO provides the legendary BMW steering feedback, whereas the F30 steering is very light (almost numb IMO). From a power/acceleration perspective, both should be similar, though the earlier N54 engined cars kick more ass. From an fuel economy perspective, the F30 is better (at least per EPA), and also from an interior perspective it seems to be more luxurious (not necessary better materials ). Exterior looks are subjective - I especially like the Estoril Blue M SPort cars - they are really pretty.
It really is a very personal preference oriented thing - I would suggest you drive both and make up your mind. For someone like me, I would find it hard to go from E90 to F30, but as you will see here, there are a few who will swear they prefer the F30.
So go drive both