Ditto the oranges to apples comment.
BMW makes sport sedans with the exception of Z4, whereas Porsche makes genuine sports cars - different approaches to defining "sportiness".
I respect Porsche making the 911 with theoritically wrong engine placement one of the most successful race cars the same way I respect BMW achieving a 46/54 weight distribution with the front-engined E30 M3. Both marques are great at what they do.
The only head-to-head competition the 2 companies had was probably the 850CSi and 928 GTS of the 90's. Bring on the fight between the new M Coupe and Cayman S!