Good Question.
I have two boards of approximately that length and use waist belts and a 9' x 9 mm leash by LeashLok Hawaii.
leashlokhawaii.comI have been using these leashes for about 3 years perhaps 100 sessions a year and no issues. You do need to check the grommets that connect the leash to the belt and the rail saver periodically as they can work themselves out with repeated use. I do take good care of my leashes and check for cuts and wear periodically. I think if you are surfing in waves that are above head high the waist harness is a good idea (or if you have wonky knees or hips).
There have been several good threads here and on the Standup Zone on leashes if you search for them.
Cheers
Bob