Macatawa-
For your draw I would go with the 70", you could shoot the 68" but the 70" will be perfect.
Someone told me this one time about long limbs:
They said, If two kids are on a see-saw, and one of the kids is fat, it would be much easier to pickup the fat kid with a longer see-saw then a short see-saw.
Howard Hill talks about longer limbs on his bows in Hunting the Hardway. His book has lots of useful information; it covers the bow design, his style of shooting and hunting. I've had my copy for almost 20years and I still go through it and reference what Hill actual said.
Darren