Well, it certainly isn't an easy question it seems! Reminds me of doing astronomy, getting good results starts to go sky high (ha ha) both in cost and size, very quickly. I do care about getting the top quality I can (with the high quality of rest of my equip I'd rather not bother with mediocre results due to issues with my camera or lens). However I would like something to see how serious I am about wildlife, if I like it then when I retire eventually I could pop for one of the big white trumpets. Though speaking of which, I'm not sure how my wife would react to a super big, white expensive lens like that. Probably mutter something about 'old rich white guy'.
Anyhow, so the 2X quality is maybe questionable, it is $500 after all and is only useful for the 70-200. Plus like Mr Mollusk I'd probably prefer a 2.8 lens in this application, if I could get it. My biggest problem is that I don't want to buy a 7D just yet, so would be using with an old Rebel which surely wouldn't focus with this combination.
The 100-400 is too old a lens for me, I've been waiting for an update, will probably wait for a good time longer, who knows.
The 70-300 has a lot of nice features (weight primarily), but doesn't give me a lot of reach and is mostly redundant with my 70-200 AFAIAC.
The 400 looks quite good, other than being slow and old (no IS or weather sealing) and the fact that I have to carry another lens around. However I do spent a lot of time at the ocean and have wanted to get shots of ocean critters - seals, whales and what not, so the lack of weather sealing is an issue here. I ruined a 50/1.4 once by taking it to the beach.
On the Sigmas, those look pretty nice but you know I just want to stick with Canon. I'm at the point where I don't want to bother with off brand gear (which is why I returned my off brand battery grip and just bought the Canon, because the buttons sucked on the off brand)
Putting all that together, I guess I don't have a good option without the updated 100-400, Gah ...