Assuming good lenses and film, a two, four or eight times lifesize image will contain more detail than a lifesize image. The only catch here is the depth of field and diffraction effects. This affects landscapes and portraits as well but I think the extreme DOF at close focus distances makes the problem worse for that work than for landscapes.
Originally Posted by BetterSense
I'm trying to think if you'll have 8x narrower DOF at 8:1 or more than that. If it is 8x narrower will it be less obvious in a contact print than the required 8x enlargement of a 1:1 image? Is there a nonlinear factor which won't come out in the wash?
Grain and tonality will be better on the larger piece of film. If the other factors can be practically overcome then the larger format should win but can you buy or make an 8:1 setup for an 8x10 camera?