Well to start off, the E39 M5 and E53 4.6/4.8 are totally different vehicles and serves it's different purpose as you may already know.
I've driven the E53 4.6/4.8 and are awesome SAV's that's a blast to drive on long trips, very comfy and with more than enough power that you'll need.
I currently have both the E39 M5 and the E53 4.4i and very happy with the combination specially here in Pennsylvania where we get a fair share of bad winters...
The E39 M5 is a monster with it's max torque available in low RPM's, it can get you in trouble really fast... Very nice family sedan when with it's comfort features and very nice full leather interior...
Let me know if you have specifc questions.
