ditto with Kuan.
the priorities for Benz are economics and packaging. For reduced manufacturing costs, they have adopted a "modular" engine design, sharing manufacturing facilities with their V8s. And V-sixes make more efficient use of the engine bay space than an inline-6.
Benz and BMW have different design philosophies. A purist like me would be attracted to the BMW's inline-6, manual transmissions, and legendary steering and handling. Benz however, tends to see its customers differently, delivering a total package consistent with its traditional values of safety, reliability and longevity. This is for customers who are more interested in the total package than the technical and engineering details.
BMW goes for more power at higher rpm, typically with a smaller engine. Benz goes for low-end torque. BMW has a chassis that wants to dance. Traditional Benzes respond with measured responses to your inputs, not unlike a butler.
these two companies know their niches and play their cards. The interesting thing is that the latest BMWs have become more Benz-like (i.e., softer), and the new C-class for example, has become more BMW-like, in terms of handling. And Benz has (almost begrudgingly) reintroduced its manual transmission.
my ideal car would probably be the C36 with a 6-speed manual. Best of both worlds.