I am a lifetime BMW owner (3 V8 5 Series cars, 2003 E39 540i, 2009 E60 550i and current 2012 Dinan Stage 2 550xi) and I just went through this M5 vs. E63 S decision myself and I went for the E63 S.
I decided on the E63 S for a few main reasons:
1. Raw power and acceleration: the M157 5.5 L V8 engine in the E63 is considerably more powerful than the 4.4 L V8 in the F10 M5. And the tuning potential is far greater. With a $3000 AMS tune or $3500 Renntech tune, the car produces 700 (AMS) or 701 (Renntech) bhp and a pavement ripping 830 (AMS) or 839 (lbs-ft) of torque. Renntech: http://www.renntechmercedes.com/www/node/1298
Even with all Dinan power mods at a cost of about $10K installed http://dinancars.com/product/f10-m5-...ries&mid=1163/
, the M5 will produce less horsepower (689 bhp with Competition Package) and dramatically less torque (668 with Competition Package) -- not breaking even the 700 lbs-ft level. With the tune, the E63 does 0 to 60 in 2.9 seconds and the quarter in about 10.9-11 seconds. http://www.automobilemag.com/reviews...rmance-review/
A tuned F10 M5 (meaning one with bolt on mods -- not upgraded turbos) does not get close to those numbers.
Same with stock power. Best 0 to 60 time clocked by a car mag for the E63 S is 3.2 seconds while the best F10 M5 time is about 3.7 seconds. Bottom line: from acceleration from the a stop, the E63 S's combination of more horsepower, more torque and all wheel drive wins this area by a mile over the F10 M5. At high speeds above 110 or so the M5 will catch and pass the E63 S due to the greater high speed efficiency of RWD but I never drive those speeds.
2. The traction of all wheel drive. I live in the New York City area. I am a one car guy so this will be my daily driver. I just do not like driving in snow in RWD cars even with dedicated snows. My V8 BMWs with RWD were a handful in the snow even with dedicated snows. No such worries with the E63 S with dedicated snows. Moreover, with the M5, if you accelerate too hard from low speed you can get the traction control light coming on and slowing things down. With the E63's AWD, that doesn't happen. Just point the car and floor it.
3. The E63's engine and exhaust noise is a lot louder and more aggressive than the M5 which needs to pump in sound through the stereo. Love love love the rumble and roar of the E63 S.
4. I felt that the E63 S felt a lot lighter than the M5 which, to me, has a heavier feel (I know the M5 is lighter). Perhaps I like the steering feel of the E63 better.
That being said, the M5 has some advantages over the E63 S but they did not win the day for me:
1. Price: while the MSRP is more the lease deals on the M5 are better than the E63 S.
2. Looks: I think the M5 looks better, inside and out, than the E63.
3. Electronics: M5 has bigger nav screen and MUCH better sounding stereo.
4. Paint: M5 has some beautiful paint colors available. I find the E63's colors to be more dowdy and conservative -- no color like Sakhir Orange or Monte Carlo Blue for example.
5. Transmission: The M5's DCT is significantly better than the E63's MCT.
Perhaps the most comprehensive comparison of the two cars is found here: http://www.motortrend.com/roadtests/...ck_comparison/
Both are great cars. You cannot go wrong with either. I got a killer deal on my E63 S so PM me if you want some information on it as I have a hookup with a great dealer who discounts the car a lot.