I also have a FBO e92 m3 DCT and the decision really comes down to whether you want a "sports" car or a luxury cruiser. The old decision was much more difficult with the e60 as it was not much bigger than the e90 and the engine delivery was the same etc. I don't even think you can compare these two.
If anyone has Fbo (save convertible guys with 400 extra pounds) you know that if you drive it properly with revs up in 6k and with a DCT, this thing is an absolute rocket ship.
If you are a "lazier" driver in terms of not wanting to keep the revs in the 6k range when looking to flog it, for whatever reason, than the m5 will be a great car because you can get thrust at any rpm and any gear. If you are not in the right gear in the m3, the thrust is not there.
The F10 is just absolutely huge. I would never give up my e92 for an f10. Maybe add the f10 but stilld ebating