I am moving from an E46 M3 convertible. It is a great car in either coupe or convertible form. It is fast and fun and practical, but it is not a sports car in my opinion. I test drove a new 987 Boxster S. The boxster is a whole lot more sporting fun. It feels quicker, handles much tighter, and sounds dramatically better. The M3 felt like a truck after driving the Boxster. Granted I have the convertible, which is not as stiff as the coupe, but I suspect the difference will be that much more dramatic with the Cayman S. I believe the 0 to 60 time (as tested) is 4.8 for both the Cayman S and the E46 M3 coupe, but I guarrantee you the Cayman will feel faster (and sound much better). Not to mention, the handling won't be close.

If you value the back seats, then no contest, go with the M3. Otherwise, if you are looking for a fun, rigid, fast, amazing handling sports car, go with the Cayman S. Of course, it will cost more and of course I have never driven one.

Just my two cents.