Hi Naturac,

I have never used the 400 f/5.6 but have owned the 100-400L for ~ 1.5 years now and have probably taken ~10,000 pictures with it. I have not observed any dust issue with my lens. I believe I benefit from the IS. It may only be two stops, but I can shoot sharp photos at 1/160th-1/200th of a second (I've even kept a couple that were shot at less than 1/100th, but those are the rare exception). Looking at the reviews, the prime does seem slightly sharper, but that is mostly at the edges of a FF camera. The center seems equal. If you are shooting a FF camera, this will be an issue. However, it is much less of an issue with a crop sensor camera as the smaller sensor uses more of the center of the light circle generated by each of these lenses, not the outer edges displayed in the tests with the FF camera. The friction collar is a little funny, but works well. I find I have to adjust it fairly often, but I do not consider this to be a problem. I've gotten used to a pump action and in some ways like it. I also think the quality of the 100-400L at ~100 mm is underrated. It is actually sharper at 100 mm than it is at 400mm and while I also tend to shoot this lens 90% of the time at 400 mm, when you need a shorter focal length, it is great to have, so I find the 10% of the time well worth it.

My last thought on comparing the two lenses is simply to evaluate them as part of your overall kit. Do you already have the 100-300 range covered? Do you need it covered?

On the canned air vs the rocket blower, I'd be concerned about the pressure from the canned air pushing something further in rather than just off.

Hope that helps,
Brant