Print films generally come with ISO/ASA ratings of 100, 200, 400, 800, 1000, and most recently 1600. The higher the ASA number, the more sensitive the film is to light, which will allow higher speeds which in turn reduces the probability of producing blurry pictures. But nothing is free – there is a tradeoff for that higher speed capability. The higher the light sensitivity, the lower the image quality will be. Films with ASA ratings of 200 or higher (dubbed “fast” film) will have noticeably more grain, less sharpness and poorer color performance than ASA 100 or lower (called “slow”) films. Aviation photographers will almost always prefer image quality, so ASA 100 is the film of choice for them.
Slide film also comes with different ASA ratings. As more professional photographers (not just aviation photographers) prefer to use slide film and often desire the highest possible image quality, slide films with even lower ASA ratings (and higher image quality) are available, the lowest ASA rating commonly available being ASA 25.
BartiniMan, the reason that you see the higher ASA films as more expensive is that most people are looking for print film for their pocket cameras, for use at home or at the beach, often with a flash. They could care less about image quality. As pocket cameras generally have small lens apertures, speed is at a premium. So 100-speed film would not be the best solution for them.
But if you are trying for a higher level of photography than party pictures, I would not go any higher than 100. I only use 50 and 100 myself.
The only thing you should feel when shooting a terrorist: Recoil.