Just out of curiosity what is the difference in price? I'll bet that the 335 will be discounted a bit more being that it is the last of the E92s vs an F, and that would make your decision a bit easier. I also heard that the N55 is a significant improvement from the N54 in terms of reliability.
I agree with Matt, the 335 is a totally different driving experience. The Fs are a bit smoother/softer and the electric steering is totally different. No way to tell unless you drive em, but I'll bet once you get in the 335 you will want it. Leather is nice but to be honest Premium is not a must have for me, the leatherette is really not bad at all.