My 335 has adjustable side bolsters... cant even get those in a M5!
In all seriousness, I had planned on getting a F10 550 this past year, but after multiple test drives, it just didn't feel as fast or connected as my wife's 2009 E90 335ix (no frills version). At the end of the year, I opted to get a new F30 335 sport, which is great to drive, and has many of the appointments previously only seen in the 5 series on up....
but I will also be ordering an M5 this summer. There are quite few reasons for this, but my main reason I want more luxury(******* and performance.
If you are serious about either car, tell your CA your exact dilemma. He should let you take either home for an hour or two. Drive them and see which fits best!