I rode the 71 yesterday with underpowered 5.0 in twin fin mode. I'd say it was nowhere near the limit. The board felt really stable and surefooted bot in a straightline and on a wave. But if you're after a more lively type of feel in underpowered wave sailing and do a lot of sailing when there are no waves, I think that maybe the 76 is the way to go after all.
As I said, it's a tough call since both boards will do the job, just with a different flavor, so to say.