I really suggest you talk to your dealer to let you demo one for a few hours. I was torn between f10 M5, 550 with a tune. I had a 550 for a few days when one of the cars was at the dealer and the dealer gave me a ride in the new M5. I was unsure about the size of the F10 but it was really nice inside. I did test drive an M3 DCT which i test drove for almost an entire day and I can tell you that day, i learned an M car is much more than just HP. If you just want a highway rocket, 50 with Adaptive Drive and a good tune, your close to 500hp and a car that is very nice and can handle the curves quite well. You also get the lovely 8sp trans which in my eyes is perfect for the 50. In my eyes, the M cars are much more raw and a lets you know it is not just a regular car. I know the rear suspension carrier is different than the non-m X cars to improve the handling even more than just the AD. I wanted a X5M with third row and found out the rear suspension on the m is different and would not accommodate the third row. There is a reason why they say "///M the most powerful letter in the alphabet" it is in a class of its own.

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2012 X5 50i ZAP(2VA, 336, 465), ZTP, ZPP, ZPS(6VC), ZCW, ZRC, 322, 4UB, 4NC, 610
2011 ///M3 Silverstone II
2011 X5 50i(lemon)
-- SOLD --
08 550 Msport
07 X5 4.8I sport
04 545i sport
02 4.4i sport
01 540I M/sport
00 SL500
97 SL500