Just_me

It's when you see these car next to each others the choise gets easier. The M5 gets my attention, more details. Better front spoiler, better rear diffuser, better looking sidemirrors. M5 looking more like a race car and RS6 looks kinda bland. Even the RS4 is stealing the thunder from RS6.
RS6 has a very intresting engine but exterior wise, the M5 is a clear winner.

bmer

I'll have to base my opinion on the pictures as I haven't seen either one of them IRL. Basically I think the RS6 looks more beautiful and the M5 T. more dynamic and sporty. So it's a draw in my eyes.

Matt

The RS6 definitely has the "bitchen" factor going for it. The car looks pure evil from all angles-like it eats M5's for dinner. The proportions are more masculine on the RS6 as well.

However, the M5 wins on the details. Its frontal expression more menacing-and thought out than the RS4's. The lights better detailed with more thought than merely a patch job on an old design. Its tail lights and shoulder line all flawless parts of a very flowing design....

Yet where M5 design has finesse the audi has pompous vulgarity-very cool. Therefore in terms of design, i'd say audi.

But you know i have to have RWD.....