Hey,
The 635CSi has 220BHP, and the M6 has 286BHP. That's a big margin. Even so, the 635CSi is a beautiful.. Beautiful car. My father has an '87 model, with only 100,000 miles on it. That said, I have never seen/driven an M6, so I wouldn't know.
However, I have the perfect site for you. This is a review comparing the two models.
http://www.bmw1st.com/6series/e24fac...4-face-off.htm
Whichever you pick, enjoy it; they are both great cars.