I find that it takes me a while to get to know a new mountain, so I'd ski at Keystone both days.
I think Friday will be less crowded than Saturday. I'd start as early as possible and take an early lunch so you're skiing and not eating at the peak of the restaurant crowds.
There are nice blue runs around the Montezuma chair; we always find the lines are longer on the right (as you face uphill) than on the left.
If a line is long at any chair, DH and I go in the singles line, which usually moves quite quickly.
Check the grooming report if moguls matter to you. Bighorn is blue, but it's a long bump run; Starfire and Diamondback are black, but almost always groomed.