I don't wear "snowmobile boots"- I have had too many friends with Klims and HMK etc. have wet and cold feet. Plus, I prefer a higher upper than any snowmobile boot has for those times I get into overflow. I have yet to have wet feet.
I wear Baffin Barrows and/or Baffin Fort Macs. The Barrows are rated to -150F. The Fort Macs are rated to -70F. I prefer my boots to have a rating at least 3 times colder than what I actually expect to ride, because I tend to get cold easy. I wear a single layer of Smartwool socks in my boots, and I don't use handwarmers in them. Both the Barrows and the Fort Macs are 100% waterproof from sole to the top of the upper, and the uppers go nearly to my knees. The Barrows upper is about 1-1/2" taller, and it is a bit heavier boot.
The downside (for some) is that both the Barrows and the Fort Mac's are a heavy boot and they aren't as stiff as a snowmobile boot is- but the weight doesn't bother me, and I've never had a problem moving around on my sled.