I think about a few main things (not necessarily in order of importance):
- What sort of sharpness and grain I want for the picture
- Whether or not I will be hand holding for the shots
- What sort of light I will be shooting in
- What sort of dynamic range I want the film to be able to capture (i.e. SBR)
For much of what I shoot in 35mm, hand held in a variety of situations - often dark, or in and out of the shade, with moving subjects - a 100 film won't cut it in terms of stopping action, depth of field, or simply being able to get enough exposure at all. 400 and Delta 3200 - which is actually ISO 1000 - are my two main films. I use slower films sometimes, but rarely, in 35mm. Occasionally I will shoot a roll of Reala, or a consumer 100 or 200 film that someone has given me. I also shoot a roll of T64 when needed, and I use Acros for copying text and other flat things on a copy stand. For 35mm shooting, I rarely need or want the technical qualities of slower films. I'll usually just go to medium or large format if I want a really sharp and grainless picture.
In medium format, I shoot more color. For hand held pix, I generally shoot NPH/400H or NPZ/800Z, Tri-X 320, T-Max 400, HP5, and sometimes I will shoot the NPH and the other faster films on a tripod. However, generally medium format on a tripod is when I will use 100 films, such as Reala, Acros, T-Max, FP4, Plus-X, or Acros for long exposures. I also use Tri-X 320, HP5, and Delta 3200, even on a tripod.
With large format, I generally use a 100 film, and sometimes slower. I use 320 and 400 (b/w or color) films as well, for faster shutter speeds, less contrast, more D of F, etc. FP4 is my main b/w sheet film.