Don't want to muddy the waters, but I think you're right in the middle. I have a 28-inch draw and find a 66-inch Hill more pleasant to shoot than a 68-inch. Don't notice any difference in smoothness. To get the same draw weight at a longer bow length, they need to make the limbs slightly thicker. Don't know exactly how much this impacts smoothness, but it seems to me it would be a wash. Type of woods combined with the bamboo, if any, glass vs carbon on the back, etc., probably have a greater effect on smoothness than two inches of bow length.
I also find the 66-inich more comfortable to carry/ navigate with in the woods. One wouldn't think an inch on each end would make much difference, but I just felt a little more awkward with the 68-inch. Really a personal preference thing. I highly recommend shooting one of each if you can.
Indie, for your 31-inch draw, I'd recommend as long as Craig makes them, which is 72 inches I think. Talk to him about it.