HMMM, Jackson Hole in January-----1st, make sure youhave good cold weather clothing, boots, gloves etc. As far as lenses go, get as much reach as you can afford. If long lenses and proper support are not in your budget, I would suggest renting the 400 5.6 L prime. Its easier to carry than the two you mentioned, is crystal sharp and most of the unfrozen wildlife will be fairly distant. Also, the push-pull zoom on the 100-400 takes some getting used to and offers lots of opportunity to miss your shot if you are not practiced with it. If you are going to use extenders, stick with the 1.4.---youll be happier with your image quality, but remember that you lose auto focus when using an extender on any lens with a max aperture at 5.6 or greater on non-1 series bodies. I think this is true even with the 7D. Perhaps you or someone else can confirm this.