Alpina and M offer different products to different clientele, if you only looking for fractions of a second on maximum attack performance numbers then obviously the M is the way to go, but if you're looking for something a little more refined and possibly more comfortable and easier to live with then you're probably an Alpina type of person.
Personally, I'd probably take an M6 over a B6, but, if they make a B6 Gran Coupe I'd probably take it over an M6 or M6GC. Not that I can afford any of them.
I do think Alpina have lost some of their exclusivity, the range seems to be a little dull these days, I'd like to see them doing something a little different... having said that the B7 is pretty much at the top of my lottery list, and I'd love to have an Alpina E24 or E28.
3/15 to i8, and everything in between (yep, even including the F26 and F45)