im with TJ, if i were trying to stay wram i would go with the Flows - most material between you and the elements. maybe even over-size them and rock the injinji socks too
ive heard the ones made for water (im pretty sure its the flows mentioned above) keep your feet warm. and Im pretty sure the chart is correct. although it is probably a good idea to go to your nearest store and check in person.
wrt sizing, yea, the only way to get a 100% perfect fit is to go try on different sizes. i realize that everyone cant do this, but it is really worth it if you can.
I wore the Bikilas all last winter (in PA) through some prett nasty snow and ice and generally miserable wintry conditions. My feet were fine after a couple weeks of acclimation. My guess is that there's not too terribly mush difference from one style to another. Your feet will be doing their best to keep warm. Now, if the Flows are waterproof, maybe that would tip the balance.
