I was in the same agenda delima as you... but I decided to go with the 6x8 punch, I am in love
!
With the new role in my company I needed an agenda, and because I am more of a visual/creative type, a palm or blackberry just won't do (I need to draw and doodle and sketch!). AND, I write HUGE, so a 6x8 is perfect for me. Also, it looks so sweet tucked under my arm when I rush from meeting to meeting, kwim
During the week I carry my biggest bags to hold it in, but the 6x8 does fit in my Legacy Shoulder perfectly... see here...
http://forum.purseblog.com/coach/i-finally-bought-6x8-agenda-punch-pix-attached-215160.html
But, it is kinda on the heavy side, so I took out the address book.
Good luck on your purchase!