Disclosure: I haven't had any training since my Hunters Ed course 20some years ago, which while fun, was almost useless as far as teaching any kind of hands on safety due to the large class sizes. Thankfully I had learned firearm safety years before that from my stepdad.
For what it's worth, to be honest, I think for me it would depend on what kind of course it is. If it's an information session (the "bookish" stuff: current laws, etc) I would say group is fine. However for on range training (safety, shooting, drills, etc.) I would much prefer a one on one (or at most a *small* group...say 3-5) where the student knows s/he is getting your full attention versus having you spread around to a group where something (not necessarily critical) gets by and at the least will probably turn into a bad habit with the potential to turn into a dangerous habit.
I'd rather pay a lil more for a class that I *know* Ill be as competent as I can be with 1-1, than save a little and not feel as confident as I could.
Sam, just my $.02