Over the past few years I have shot a handful of airshows, first with the t2i and a 70-200 + 1.4x (blue angels below), then a few with the 7D + 100 -400 and even with a 6D + 100 - 400 (thunderbirds below). For the upcoming Chicago Air and Water show I have been thinking whether there are any equipment upgrades I can make to increase the quality of my images. The 100-400 is by and large THE airshow companion for good reason, its reasonably priced, flexible (wight and zoom), and has IS. I have been considering picking up a 400 5.6 for some time now to see whether I like it better than the 100 - 400 or not; though the IS in the 100-400 is key for panning shots. However, today I got the idea to maybe rent a 300 2.8 IS II with a 1.4x or 2x. I am not normally a fan of renting; I'd rather take the money and spend it on something I can keep..but that's neither here nor there.
Obviously shooting airshows can be hit or miss due to perspective, lighting, weather, and atmospheric conditions..luck...
So the question is what are the advantages of using say a 300 2.8 + 1.4x or 400 2.8 over a 400 5.6 or 100-400 to shoot an airshow? Obviously DoF isn't an issue since you are shooting near infinity, though the lower f/ will allow you to use a higher shutter speed and/or lower ISO. The real difference is probably in the sharpness; so the question is to anyone who has used a lens from both categories, is the IQ gain really worth the hassle of renting/twice the weight? I know for birding the IQ gain is very noticeable, but for airplanes?
Thanks in advance.