Never understand why they would take such a performance looking car as the SL and put a 6 in it. To me it's like buying a Mustang, Camaro or any other sports type car. 6 cyl's just don't belong in them in my opinion.
Go for the 500. Nothing like having the feeling and hearing the sound of over 300 hp when you step on the gas.
Whatever decision you make I hope you enjoy your new car.