Lens suggestion for July trip

I will be making my first trip to Alaska July 17 - Aug 2, 2009. I would love to bring all my gear but the baggage limits on the airlines require that I keep the weight down. The trip includes a couple days at Brooks Lodge, 5 days around Denali and a week on a small boat cruise out of Juneau (The Boat Company) around the Inside Passage & Tongass National Forest. Equipment includes a Canon 5D2 with 24-105 lens. The question is which telephoto lens would you recommend I bring along a) Canon 100-400 f4.5 or b) Sigma 120-300 f2.8. I'm torn between the IS and reach of the 400 vs the speed of the Sigma.

My vote would be for the Canon. IS should give you 1-2 stops, and the extra power will come in handy.

Thanks - the Canon is how I was leaning - with the lower weight also being a significant plus (even though I don't expect to do significant hiking due to my wife). The problem is I just love my Sigma lens - it is exceptionally sharp and I use it more than any other lens on wildlife.

I would agree with Jim, the 100-400 would be a good choice due to the I.S. and extra reach. However, if you have been very satisfied with your Sigma & are obviously comfortable with it - that is an intangible. On a boat though - my experience has been that the 100-400 IS really helps a lot. If only they made a 200-500 IS (f4 - f5.6)... I'd be all over that. That with a 70-200 2.8 or 4 would be a great combination... Oh well.
But to answer your question - In Alaska, you are always looking for more reach it seems
The 100-400 is plenty sharp at 300 too. Have a great time & enjoy Alaska!

I do lots of work on boats and other awkward situations that preclude tripods. Though I'm using Nikon rather than Canon, IS has made the difference too often to ignore. I usually opt for a 200mm IS zoom over either of my 300's simply because the zoom is IS. Having both IS and extra reach of a 400 would be an easy deal breaker for me, no matter how sharp the 300 (and mine are definitely sharp).