I would use a series III neck mainly because of material/finish choices, availability and price. The III neck is exactly like the late MK VI neck. At this time I am using III necks on my MK VI and Selmer USA tenors - Sterling silver and 'gold lacquer'. They are interchangeable so I swap them every so often just to appreciate the differences. Both play fine but the Sterling one is exceptional on both horns. The main difference in a MK 7 neck and a MK VI/Series III is the shape of the neck - the MK 7 has a longer straight section coming out of the body which throws the big arch higher, making the overall length shorter. The Series II is the same, but for the series III they returned to the 1967 neck.