Which telephoto?

Hey guys, I'd like your suggestions. I'm torn between getting the Canon EF 300mm f/4.0L IS USM lense and the Canon EF 400mm f/5.6L USM. They are both around the same price and I'd like the extra 100mm, but the drawback is no IS. I've been reading reviews of these two lenses everywhere and still can't decide between them. I'm going to be using it mostly for wildlife (mostly birds) photography, so the extra distance would be helpful. That said, I'd like to get most of my shots without having to lug a tripod around. I plan on using either in conjunction with the 1.4x telextender. I'd just jump up to the 400mm IS, but I'm on a $1500 budget. What do you guys think? Something better to get? Or stick between those two?

Edit: Ok, 3rd choice: The Sigma 50-500mm. Now I'm really torn.... I don't like choices, someone make it for me!

We can't decide for you. It's your pocket book.

Wildlife, or even just birds, is still a broad subject. There are flying birds that you want to snap "on the wing" so to speak, and you need fast autofocus for that. There are wading birds that just walk around (much easier). There are hunting birds that tend to be larger (and require less focal length). There are shy birds that are difficult to approach (and require more focal length). There are friendly birds that will walk right up to you.

Lately, I've gone after lots of hummingbirds, and I wouldn't even think of shooting without a tripod. Canon 100-400mm with two 1.4 teleconverters.

Green herons were good this summer. Same rig.

Hawks have been good lately. The same rig with one 1.4 teleconverter.

Blue herons are good now. The same rig with no teleconverter.

I'm still looking for a burrowing owl.

There's probably some good eatin' in there.

---Bob Gross---

I didn't even think to use two 1.4x converters!
I guess I shoulda been more clear. It'll mostly be used for the smaller, shyer birds.

At $1500 budget, why not consider the 100-400L IS?

They're not that big at all, and besides, are a protected endangered species where I live.

The prevaling wisdom is that if you start with a very sharp lens, and then you add a 1.4 teleconverter, you get a good effect. If you add two 1.4 teleconverters to a very sharp lens, you are getting out on the edge of what is good or not. If you start with a soft lens, then add a teleconverter, you get sort of a soft mess.

Where I am, it's getting toward the winter months, and there is less light in the sky. So the lens that was great two months ago is now "not so fast." This would be a big deal if you were going after Sandhill Cranes at Bosque del Apache NWR.

---Bob Gross---

I guess I can go with the 100-400, just won't be able to get the teleconverter for awhile.

However, the 100-400 in many review sites showed problems with parts wearing out and getting dusty very quickly. That's why I was avoiding it.