I swapped my 2013 330d xDrive Touring for a 2016 M235i Convertible. They are very different cars, of course, and you can't deny the quiet power of the 3.0L diesel, where you can cruise at 70 in 8th whatever the incline. So I understand the feeling that there's more gear-changing going on in the M, and on a long motorway cruise, it's not as quiet as the 330d.
I'm glad I went for the Convertible. You can drive with the top down for more UK days than you might think. My only previous convertible (Lotus Elise) had a manual top, so sub-20-seconds is a luxury and it's really not a risk running with the top down when the weather is borderline. The 5-layer fabric makes the interior sound quite close to that of a hardtop, and the open-top experience transforms a drive.
Add this to the agility and immediate power of the M235i and in my case, I'm very glad that I made the swap. Remember to get a fold-down rear seat - this makes the available luggage space much greater, whether coupé or convertible.
