70-300L or 100-400L?

I am finally done with my Sigma 150-500 (on ebay at the moment), and I am looking for a replacement.

This time I do not want to make the error of trying to be cheap, just to end up spending the money twice.

A friend bought a 70-300L recently, and I had a chance to handle it for a while. Its really nice, sturdy, and the IS seems the best i have ever seen.

On the other hand, its 300 vs 500mm at the long end, and it cannot accept extenders. Quite a step backwards, even considering that the 500 is not sharp at the long end.

The alternative would be the 100-400L. Can accept extenders, but has a much older IS than the 70-300.

From reviews, both seem to be about equal in sharpness.

Does anybody have experiences with both of the lenses?

also, about extenders: I know that the canon extenders dont work on the 70-300. I tried my Sigma 2x, and it just barely did not fit (2mm or so were missing).

I heard that there are some kemko extenders that work... is it possible to stack like lens-> kemko-> (other extender)?