Hi, has you actually gone down to the shop and try the different millions size out?
I have chosen 7 million for the following reason (now waiting for MO):-
1. Just slight diff from 6M by $10
2. It's the biggest sling bag that I can manage, even if it's empty, it's not too big for me to carry around I hope.
3. Actually like brazilian M design with all the extra extra, but that's too much.
4. I could put in other stuff, if the camera cannot fill it up. If too tight fit, then any additional will need extra baggage.
So, unless you r thinking to change later, I think to have something that's bigger enough and just nice to manage like a normal bag. After all, choosing a Crumpler is that it's fashionable to be causally carried around even with or w/o filled with camera stuff. Unlike a camera bag like lowepro, telling people straight that "Hey, I'm equipped with camera."
Just my personal view for your reference only.
Cheers!