If they do chose Vee or in-line it will be all to do with packaging. Smoothness can be cured easily and I doubt there would be an output advantage of having a vee over an in-line setup.
If the choice of how the cylinders are arranged is your sole concern and reason for buying a car then you are not the right kind of person to appreciate these cars in the first place.
P.S.
I've just read the latest EVO mag just on sale today and they have run an article on the future M engines, guess what, they seem to be of the same opinion as myself that the next M3 engine will probably be a V6 because of packaging.