True a bow is more efficient with the limbs bottomed out and this leads to greater arrow speed with a given arrow weight. In your case5# will be barely perceptable. You can gain maybe 1 or 2 fps shooting the 65# version vs the 70# backed off. Very little to worry about. I would recommend you get whichever you want.

The only thing that I would take into consideration is that it would be better with the 65# bow if you were to want to back off a little more at some time. Maybe as you get older or maybe some shoulder or neck injury.