Bazz - I can't comment specifically on the F10 and F36 comparison, but having and having had both 3 coupes and their "corresponding" 5's over a couple of generations, often alongside one another, I can say that there will be a noticeable difference in refinement. The 3's have always been a bit more driver focused and their size and weight advantages will usually give then an edge in outright handling (a good thing ) but the frameless doors and lesser sound deadening, etc. will mean that they have more ambient noise (road noise especially) - but this will be somewhat offset by the more pleasing and refined 6 pot in this case. Suspension will also be a bit more harsh and unforgiving, especially on RFT's, compared to a 5 but that's the price of being a bit more nimble. However, the extra weight of the 6 pot at the front becomes a bit of a double edged sword in that it may also make it feel a little more inclined to understeer earlier, maybe compared to a 4 pot version. The offsetting benefit of course is that it has the torque to be able to drive around that in many circumstances, although the auto-only spec may compromise that a little.

In terms of build quality, fit and finish, etc., my experience has been that the 5's are better put together, are significantly more cosseting overall and generally feel more like they are hewn from rock, especially on longer runs. We drive from Tipp to tiptop Donegal fairly regularly and the difference over a non-stop run like that is quite noticeable by the end.

A mixed bag of feedback here, so it really depends on your personal priorities. You'll definitely be more involved in the driving experience of the F36 but there is a price to pay. For the comfort and refinement of the 5, you'll obviously pay through it being a bit less communicative when you want to press on or are driving with entertainment being the ultimate priority. The 5's aren't shoddy in this way, just that the balance is tipped a little to the other priority.

I've contemplated a similar choice recently and my own conclusion was that I like the look of the GC and they are pretty good value in 430d spec, if shopping across the border or the water, but I personally feel that after a while I'd be looking at the refinement deficit and how to fill it. The answer might be a 530d in F10 m-sport spec. Caveat - my view on this could be shaped a little by having the option of wheeling out a coupe when the urge takes me.