Do you like piecing on a 9mm machine? Both the 750 and 780 have 9mm max stitch width with dual feed. My Bernina 820 is 9mm with dual feed, and it's excellent at piecing long strips and most pieces, but when the pieces get smaller, I prefer a 5.5mm Bernina. Not everyone minds the 9mm for piecing, but some do.
The 3 series, (older) 430, 440, 630, and newer 530, 550, 570*, 720 and 535 have 5.5mm max stitch width. So the feed dogs are closer together and presser feet are more narrow. The older 435/450, 640, 730, 560, 580, (newer) 710, 740, 750, 780, 790, 820, 830, and 880 plus the very new 570* and 590 have the 9mm width.
Confusingly, there's a not-that-old 570 with 5.5mm stitch width, and a very new 570 with 9mm stitch width, dual feed, and hover.
The 7 series are fairly large and heavy, probably not the ideal choice for classes.
Just stuff to think about...good luck shopping!