I also have the 100-400, and it was my first L lens. Personally, if I was to do it over, I would get the 400 5.6. I tend to use it at 400mm most of the time anyway, and at 400mm it would be on my tripod which negates the need for the IS. So I would rather have the added IQ of the prime. I would occasionally miss the range, but I would rather fill it with a 70-200 f/4 IS (actually the f/2.8 II, but that is a little out of reach right now).

Based on some of the shots John has posted, his is a very sharp copy of the 100-400. I was not as lucky. I found out how bad mine was this past spring. While in the Smokys I dropped mine on a rock (don’t go on vacation with a new ball head that you are not use to). Basically the body cracked and dented, but the glass was intact. So after $200 in repairs, the glass was reset in a new body, and it was the best thing that ever happened to the lens… it is much better now.

As for the push/pull thing... that seems to be a big issue more by people who do not have one. It may take a little time to get used to, but I have never seen it as a problem, and as John pointed out, it does make it compact. I also think the friction ring works OK - I would rather not have to mess with it, but it is not a problem either. I have not experienced a problem with dust either.

If you are going to use the range or hand hold it, then the 100-400 IS would probably be better.