I love my M6 The box takes a bit of getting used too. It's not exactly an autobox, but can act like it to some extent. It's took me over 400 miles of driving to really understand it.
The drive is amazing and i've never set in a more comfortable car. You can switch everything off if you want some fun time In fact you can set it to adjust the suspension to your set preference, shift rates etc all at the push of one button, not to mention the extra 100hp is gives you
Practicality isn't great if you're talking about the back seats. It's not too bad and depending on how tall the driver is you can fit two adults in the back, but ideally no one over 5'6". The M5 of course is better in this department with proper back seats, but loses boot space and it's ugly as @*!#
Having never driven an M5, I can't compare the 2, but as you said, the M6 is lighter and has a lower COG, so may handle a bit better. But considering you would have to be one hell of a pilot to even come close to out performing either, it doesn't really matter which handles better because the M6 is so much better looking