Common sense would dictate that the more Thinsulate there is the warmer the boot should be. However, as someone mentioned above, that is not always the case. If the boot is advertised as waterproof even if it does not have Goretex then I would still buy them. I have at least two pairs of boots that were advertised in a like manner and I have never had a problem with my feet getting wet while using them. Ofcourse, you can also buy waterproofing spray at any hardware store for added insurance.

Make sure that there is plenty of room inside the boot even if you have two layers of socks on. Warm air gets trapped in that small "pocket" area between your foot, the boot and the outside layer of the boot. That is what keeps your feet warm. Ofcourse, make sure you have some good polypro liner socks in conjunction with regular wool socks and you will be fine.

Lastly, since everyone else felt the need to mention suggestions for further boots then let me suggest Arctic Shield Boot blankets. They fit over just about any boot and I guarantee you that your feet will never get cold.