5mm drylock split toe for No. Mo. Co. for a compromise between warmth and performance. After a few hours the pinky toe and heel start to get numb, but by then I'm done anyway. Longer sessions? Go for the 7mm.
Smartwool snowboard socks under the boots help keep you warm and make getting in and out a lot easier. Something like this...
http://www.smartwool.com/mens/socks-...ight-1200.html
Been wearing the same dedicated pair for 5 years and they wear like iron.

