Actually, I think you'd be better off with one off with a larger camera to do demos with. The screen is much larger and it is easier to show what you are trying to demonstrate. A smaller camera is useful since it is more important and harder to focus especially into the corners, but the whole plate or 5x7 will be alot easier to show the students what you are doing.....the 14x17 is maybe a little too much like a big screen TV...(:
I shoot 8x10 for b&w and 4x5 for color. I much prefer looking at the ground glass of my 8x10. I just wish 8x10 color film was cheaper!