For a loaner revolver, I'd look for a used S&W M10. -4" Some police trade ins im my area have been priced right.
For a loaner semi, after much thought, I have concluded that a Glock 17/19 is the easiest system to teach. No decocker, hammer or external safety. But I do not force autos on newbies. HOwever, the above described, striker fired, poly pistols can be safely taught with apt students and a capable instructer.
I like the Glocks cause their simple and tough, but I suppose the MP and XD family would qualify as well.