Mike,
My dad has an '07 M5 and I have an SPF with a 402R Roush (515hp). My family over the last 5 years has owned/own a 335 (mine), 325, 540, 750, M6, M3 as well as some repeats. You can't argue with German craftsmanship and the performance numbers on the M series makes me weep for Maserati (which I also prefer to any Beemer).
The M5 is scary fast BUT, it is so refined and sterile that in my opinion it simply isn't fun to drive, like making love to a robot (a sexy, well built robot). I'm not exactly sure what my Cobra will do 0-60 (especially with those BF TA's
), but after mine and my dad's swap, my dad couldn't believe how fast the Cobra was and I couldn't get out of the M5 fast enough (5 series also have pretty garish interiors imo).
Keep the M5 for every day and the Cobra for fun. There is nothing like it, such a visceral experience. Off the line is tough with over 500 HP but 40-100mph, nothing can touch me, except a better power to weight ratio.