At least for Rome Total War and Medieval II, experience does mean a difference. A veteran army will fight for several times longer against overwhelming odds than a newly recruited one before breaking. It also depends on training: even green Roman troops will last longer in prolonged battle than green Gallic ones. Do not remember how later games deal with that. One thing the author of this series stresses is that battle is very emotional and you can't learn from a book that running away is a bad idea.