ORIGINAL: I have not tried the HS 56s yet. The HS 65s are pretty heavy, but so far have worked flawless.
Personally I love the HS65s. They're nearly the same weight as the HS56s and have got 25oz/in of torque vs 15oz/in for the 56s. They're a little more expensive but I have yet to strip any teeth on a 65, whereas my pile of gap-toothed 55s continues to grow...
EG