Although I am not a Merc guy (I'm too young to be driving in a Mercedes at age 39... maybe when I'm 45+
), I think the CLS would be a great car. I think it would be a more comfortable and faster but less "fun" car to drive than the M3. Therefore, I think it depends on what you would like... more involving and fun (M3) or more comfortable and faster (Mercedes).
Good news.... you will be driving an awesome car either way!!