I have a S5 and a X6 50i. I used to have a M3 but went to the S5. The reason is mainly the interior. No doubts the BMW's have quality interior but they are to a degree sterile. I disliked the double hump design. It made the nav. seem an afterthought. Now, I noticed the X6 interior looks much better.
I am very pleased with it. The Z4 offers the next gen interior as well, also much improved.
Having said that the M3 has by FAR a better engine. It's a sheer blast to drive. I still think about going back to one. The S5 offers a more refined look inside and out. I can't wait to see what BMW does with the next 6.
On the plus side, them competing is good for us. Better products, more innovation, etc.