They're all good, nothing to watch for. Obviously most want to ride/race the latest they can for the class they want to ride, eg: 77 in pre '78. '73 CR250 has a broader power band than '74 & would probably be easier to ride for most people, however, '74 Mag is the sort after bike. I'm preparing a '75 GP250 for pre '78, 1. because I had it in the shed & 2, I like the lower seat height. I doubt it will be any less competitive than a '77 model.
K