Here's something I wrote for fighter control a few days ago, it was mainly considering low level aviation photography - the only real difference is that your likely to be handholding the camera for a lot longer at an airshow so weight becomes a larger factor:
I've seen lots of 100-400 lenses used on the Mountains in Wales and I've used a few different ones myself, here's a mini-review of sorts which might be useful (should this be its own thread)?:
Canon 100-300 USM
This was the first combo I tried at low level, the lens is fairly cheap but has a USM motor which gives you reasonably quick focusing in good light. The max aperture size at 300mm is f/5.6 which isn't much use in the winter - the lens is also quite soft at 300 and needs to be stopped down to F/8 to be really sharp - only any use on the brightest of days.
Canon 300 F/4L
This is a fantastic prime lens and a good length to use in the mach loop. The focusing is very fast and the lens is very sharp at F/4. The reasonable max aperture means that the 300 F/4 can take a 1.4TC and retain focusing on most bodies. The real benefit of this lens is the price, I bought mine from the classifieds on talk photography for about £300 - super bargain!
Sigma 120-300 F/2.8 [Canon Fit]
Not a bad lens at all but very heavy - I mean really heavy! The image quality is pretty good, certainly rivals the Canon 300 when both are at F/4. The wide aperture of F/2.8 allows the camera body to focus the lens in lower light, making this useful for use on those darker days in the loop.
[This year I switched from Canon to Nikon]
Sigma 100-300 F/4 [Nikon Fit]
Another decent quality Sigma lens, perhaps a little more contrasty than the 120-300 and almost as sharp at F/4. The HSM motor in both of the Sigma lenses is pretty good and almost as fast as the Nikon AF-S and Canon USM systems. The lens is still quite heavy and very long but does a very good job of staying sharp all the way through the range.
Nikkor 300 F/4D (IF) AF-S ED
I once read that the only lens better than the Nikkor 300 F/4D is the F/2.8D and having seen results from both I'd probably agree. The lens is very very sharp and offers excellent contrast and colour reproduction. The AF-S motor is fast and quiet - this lens really shines when a 1.4x teleconvertor is attached* where the difference to image quality is imperceivable. It really is a great lens but its also double the price of the Canon offering. I'd say it is a little better all round but its certainly not twice as good.
Primes vs Zooms
Primes will almost always be sharper, especially when used wide open - if you don't want to compromise on image quality then this is the way to go. Zoom lenses offer more flexibility, very useful when photographing from a fixed location. With prime lenses your only choice is to "zoom with your feet" which can mean lots of moving around depending on what comes around the corner. I prefer shooting with primes because they give you one less thing to worry about, if the aircraft comes too close to fill the frame I'll move to focus on the cockpit or some other creative measure.
Image Stabilisation or Vibration Control
Depends - A 300 F/6.3 with VR isn't really going to be any use, A 300 F/2.8 VR might be more useful but I've never bothered with VR or IS so I can't give an informed opinion.
Hope this helps
good point 
) as I've been thinking of upgrading my current Nikon 55-200VR for some better quality glass.