Quietness, refinement and interior quality are what will be selling points for me. Honestly if they are not improved noticeably I will be switching to a 335I. I have a lot of brand loyalty but it has it's limits.
I have limited experience in the 335 but their potential is beastly, and the tuned 335xi I rode in was a much, much nicer car than mine in nearly every way. It had far better isolation and solidity, and hauled ass while still feeling far more refined than my speedy jalopy. This car was quiet and comfortable and ran 12.7 with less tuning than it currently has. Plus it hooked in the rain way better than my GTI does in the dry. Definitely a car on my short list.