I'm probably in a unique position as I own a '11 335i xDrive coupe. Just went on a little shopping expedition with the GF, who was planning to buy a 2012 328i xDrive.
But, to keep everyone honest, we extensively drove the '12 C350 coupe (and the E350 coupe briefly), the 328 and the A4/S4.
First up.....C350-not bad. Less than exciting interior. Probably the least sporting of the ones we drove. Cramped....felt even more cramped than my 335i coupe. MSRP was right around $54K as it was all loaded up. But, the dealer seemed more than willing to get it closer to the low end of $50K with no effort to keep us from shopping around. Didn't help. That was scratched.
2nd up....A4....really a classy car. Drove nice. She liked the styling, in and out better than either the BMW or the Benz. Extremely well put together. Beautiful inside and out. Nice balance of ride/handling....probably the best of all we drove. All the tech you can handle. Priced somewhere in the mid $40K range.
(Almost) the last car we drove....the 328i xDrive. Being used to my 335i, the difference in power was readily apparent, as was the "rougness" of the turbo 4. But, once under way, it performed well. Steering was light compared to my car. You could definitely tell the size went up, especially compared to my coupe. We drove a sport line, which I happened to really like. But, she liked the "luxury line" better. The one we drove had an MSRP close to $50K, with the dealer willing to discount a little, but not much ($1,500 off). She almost bought it.
She wanted to drive the A4 one more time. Set up an appointment, but the one she was interested in, was out on another test drive. After mentioning to the dealer I drove a 335i, he gave us the keys to one of the two S4s they had. That was it....game, set, match. It was a 2013. As much as I hate to admit it, the S4 trumps the 335i. After fidgeting with the dealer on price, she got it to a price that was pretty close to the C350 (but the S4 is a MUCH, MUCH better car than the Mercedes). She loves the S4. Then again, she loved the A4, too. S4 was in a whole other category, though.
Still love my 335i, but I drive her car as much as I can. It's just an extremely well done sports luxury sedan.