A quick answer...
MB was still fairly empty on Wed. Not the prime spots, obviously.
It is techically inside the track i.e. you have to show your entry ticket to get out and back in. I.e. if you walk in the main gate you can walk all the way down the grandstands and into MB without showing a ticket again...
The atmosphere is tremendous, especially for CA members, as there are simply so many of them there. It depends what you want, but I can't go in any direction from my camp without having to stop and share a beer or three with someone. That's my idea of a campsite!!
However there are obviously a lot of non-CA people there who also seem friendly enough.
I would guess that HA is slightly more out of hand for entertainment and fireworks etc, and that MB attracts a slightly more "upmarket" camper (if there is such a thing). More caravans and mobile homes that HA.
My experience of HA was in '97 and '98. For me, for the reasons above, MB is the place to be - but HA would come a close second!