While I will defer to the more experienced here on boots, esp. those in colder areas than here, I will say that IF you are hunting from up in a tree stand and not moving around, and the high temp for the day is 20-25 or less, it gets very cold, so I'd go with the 1600 or 2000 or at least 1200, along with the proper socks. Best socks setup is a silk underlayer to wick sweat, and a quality merino lambswool for warmth. Agree with above to buy the boots 1 to 1.5 sizes larger than normal to have room for thick socks and a chemical toe warmer. Nothing will make your feet cold faster than having tight boots - regardless of insulation.
Now IF, on the other hand, you're moving around, even just still-hunting slowly, that's enough movement, and your feet are enough out of the wind to go with much less insulation - 400, 600, 800, or 1,000 grams. And the more ergo build of the less insulated ones will aid in comfortable and stealthy walking. So get one of each to be ready for any hunting situation. Also, those wraps for the outside of your boots that someone mentioned, for once you get to your stand, should work well in theory, it seems to me.
Cap'n Charlie, those Baffin boots look like a seriously good deal. My really cold boots are the Irish Setter Bucktracker 1600s:
http://www.cabelas.com/prod-1/0020812830408a.shtml
They're supposedly rated to minus 160 or 170, but really, your headgear and torso gear is more important to how cold you can go than the boot itself, so the ratings are pretty meaningless. If you have on only boots, but are naked otherwise, I kinda doubt your feet will stay warm in under negative-150 degree weather.
For "normal" hunting temps, with daytime highs in the 30s or 40s, on up to the 60s or 70s, I use Wolverines King Caribous with 800 grams, or a pair of Rockys with 600 grams.