Depends on circumstances. Lot of caveats in my thinking. Normally better chsnce yo win prevails. But, if trainer is big $ winner, or horse is young with better future prospec ts of $ (sire stakes), or driver is committed to trainer owner (mcnair grant eg) exclusively, might not go to more likely winner.
I give considerable consideration to a top driver hopping off a horse that he would normally drive to go to horse with longer ml odds. Also some drivers stink at gauging prospects, others are very good at weighing chances. Learn who those are and it helps.