Trouble is, since the take over, Mercedes just aren't built properly anymore. I've got a 13 year old W124 which is as solid as a rock. Its built like a truck. I know a guy who's got a 3 year old that is rusting! Whats that all about?
But then, if you're thinking of buying either, your question sounds like you've got more money than sense and wouldn't appreciate either of the marques.