Question to the OP.. Will you be looking for a CPO car, or does that no matter? Will you be modding?
Since you have two convertibles in there, big performance is not really a concern. Are you looking for a cruiser or more of a high strung car?
How practical does this car need to be? The SL and M3 are def more practical than the Z06 and 911. I've had the latter two, and the 911 (turbo) at stock ride height was tedious on any type of driveway. Actually had better clearance with the short overhangs on the Z06.
Engine wise, stock, not sure what SL you're looking at, but you will have more torque than the M3. In fact all those cars will have more low end grunt than the M. Coming from the M5 where you make more torque at idle than the M3 makes max, you might be frustrated unless you are seeking to rev the piss out of it to get the most power.
Z06 has power everywhere, and gets surprisingly good gas mileage.
Fit finish, stay away from the Z. Later ones are way better.
So are you looking for
- performance (Z06 then 911)
- cachet (Porsche, benz, M3)
- Cruiser (benz, M3)
- Track toy (Z06, 911)
- All arounder (M3, 911)
- Bang for buck Z06 all day. Parts, gas, etc..
To me the M3 is too much like the M5. Becomes sort of a coolaid choice. The 911 or the Z give you a different feel, and also expand the groups of people you can chat with about stuff. I enjoy relating to domestic and European enthusiasts. But that's just me