No I do not look for another option then stated above.If you do not know enough to choose between these two radically different options, I would say it is appropriate to assume you also don't know enough about alternatives to lock them out. You are merely artificially limiting yourself to two inferior possibilities.
I do want the better image-quality!
Both the 70-200 with an extender and the 100-400mm are rather suboptimal choices for this range. Even though the 100-400mm is a "dedicated" zoom, it is beaten by the rather ancient (c. 1998) 400mm f/5.6L for sheer image quality, and the loss of the first-generation IS doesn't seem all that big a problem either.
From what I have seen, other good options include:
The new Canon 70-300mm f/4.5-5.6L gets you very close to 400mm natively. Not sure about extender compatibility.
The brand new (released June this year, or so) Sigma 120-300mm f/2.8 EX is a fantastic lens, and gets better every time I use it. I am getting even more confident with wildlife shots, even with a Canon 2X III extender, in good light. It almost works as well as much shorter lenses for close-up shots - very usable maximum magnification. Downsides are that it's a bit more expensive than the Canon 70-200mm, probably considerably heavier, and I do have some trouble with the optical stabilizer system (but maybe it's the camera). Focus problems as well though it just seems the camera I have it on is to blame (the T1i with its old 9 point autofocus system). If all I needed was the 400mm equivalent range, I would have gotten the new 1.4X extender and I'm sure the image quality would be very good still.
I would avoid the Sigma 120-400mm and probably the 50-500mm as well - I would not be surprised to learn that "budget" options like these are what set you against other options in the first place. But, mark my words, for $500 less than the Canon equivalent the Sigma takes some hard knocks, but at 2X the price of the 100-400 and a bit of loss on the far end the f/2.8 zoom is the best option in the list so far. The only lens which seems able to equal it is the recent 70-200mm f/2.8L IS II but you would need multiple extenders before you can rival what the 120-300mm can do.
I would strongly suggest trying out or renting the lenses you think may be your choice. I don't know how you would go about renting a Sigma 120-300mm.
Photozone.de's tests show the resolution falling off in the corners of a full 35mm frame, and APS-C seems very good on the other hand. Probably the ideal Canon camera for using the 120-300mm is the 7D or perhaps the 60D. With the 120-300mm f/2.8 + EF 2X III combination on the T1i (500D) metering has been inconsistent, autofocus speed and accuracy have been inconsistent (in all but the best light), and even the optical stabilizer doesn't seem to kick in when it's supposed to at times. Without the extender, it performs as it should, and all these points are much improved.
But given that cost - it takes very sharp pictures at a 600mm equivalent setting, enough so that I am willing to put up with the quirks and felling of spinning a roulette wheel when I snap the shutter. I get a good enough number of great photos that I am very happy to have the equivalent of a 600mm f/5.6 IS lens at only $3700 dollars, and it transforms into a zoom lens that is simply not equaled by any other Canon lens by simply taking the extender off.