Airport and train stations locations charge 21,5% location surcharge. Longer term rentals it makes quite a difference. I usually rent from city location (Schwabing). I think Westin Grand Munich Arabellapark has been added just recently as option for luxury car rentals (if you go there during the summer you probably understand why). If I would be staying Westin Grand or opposite Sheraton Arabellapark I would probably consider renting from Sixt Westin Grand location.
As mentioned in earlier post Sixt does not allow one-way rentals for Luxury cars. You can always try to ask as long as it's between stations that have luxury fleet.
If you fancy Porsche 911 then you could try Avis since at least in past they have allowed one-ways between stations that have Avis Prestige fleet (Porsche). I think they charged extra 25 euros for one-way between cities and nothing within the same city.
Avis has offer for 911 rental if your rentals starts between Monday - Thursday. 199 eur / day with 250 km per day included. Link: http://www.avis.de/50jahre911