Help me decide: super tele for alaska?

I'm going to Alaska for a quick vacation, planning on doing three wildlife/glacier cruises in Prince William Sound and Kenai Fjords national park. I'll be flying in to Anchorage and taking a train to Seward.

In addition to the other lenses in my sig, I plan to rent a "big white", but i'm torn between the 300/2.8 and the 500/4.

My thoughts/consideration​s so far:
half or more of my shots will be from large boats, using a mono pod as much as effective. I also plan some shore-based photos, though. Hand hold ability would be nice...300, "all day long" 500,"a few shots" is my impression.

I need to transport the lens in carry on baggage, smaller size would be an advantage. (300)

I have 1.4x and 2x tc's, which I can use.

I stopped by a camera shop today to get an idea of the sizes. (Paul's Photo in L.A.- great shop, btw.) They had the 300/2.8, which I thought would be perfectly manageable, and the 400/2.8, which was an absolute behemoth-too big for my skinny arms. They didn't have a 500, but I know it is just about halfway between these two in weight. It's hard to say for sure how I'd do with it, or how it would travel.

F/2.8 might be handy for low light/fast action shots, but I have a feeling I'd end up using a tc a lot of the time for more reach if the light was bright enough.

With a 500, I'd have lots of native reach, plus a 700/5.6 if more was needed. (native reach trumps tc use...)

300 might not be a enough of a boost in focal length over my 70-200 mkii to justify the rental, and I could use the mkii if a low light situation called for it.

The 500 is something I'd love to try...something of a dream lens, but i'm worried it will be too much to handle.

With either lens, I plan to rent for several extra days and bring it to Rocky Mountain national park for the weekend before to practice/work on long lens technique.

The main focus of my trip is photography, and I want to come back with the best images possible.

I'm tempted to just "man up" and get the 500. Any thoughts? Am I crazy? Or just slightly mad?

Thanks for any insight!

Adam

the 500 would be great. Thats on my wish list

You seem to be more interested, yet wary, in the 500. Rent it. Rent a Monopod to use with it too.

Just remeber this when going to Alaska, longer reach > all. People from the lower 48 can not grip how far away things are. Many people have dots on the image and tell you they are xxx or yyy. IMO the 100-400 was not enough reach. My next trip I will be renting the 500 or 600 with a tele. On the boats you are going to need to be able to hand hold them, mono pods may be able to be used but things are going to be tight for space so tripods are out. if you can aford one of the captians choice day trips in the small boats out of Seward (Kienai Fjords Tours) take one.

I had a great time up there and the tour was well worth the cost for me as we had a good wild life day. (lots of things came out for photos). Have a great trip and enjoy the shooting.

Thanks, everyone! I think you have me convinced! 500 it is!