Correction, the following models have the same engine as the CLS55: S55, CL55, E55 limo+stationwagen, SL55, G55 (MY05).

But I guess that only reinforces your argument.

I agree that the M5/M6 engine is much more desirable, just due to the mere fact it is a naturally aspirated engine achieving those specs. Secondly, to my experience, BMW has always produced engines which are technically superior and more smooth than the Merc engines. I'm generalizing here guys.

From a design point of view, I love the CLS. It's provocative and stylish at the same time. The M5 is more sporty and doesn't not command the same level of aura as the CLS.

It all depends on what your preferences are. From a performance point of view though, let there be no doubt... the M5 is the winner.