Thanks for the replies. This is making me think that I should just do the two nights at Tambo.
I was thinking that maybe I'd like to see some of Cusco but if I'm going to be too tired and it's going to be late, it certainly doesn't warrant spending the night there just for that.
I suspect the airport transit issue is just a longer ride in the morning but I've got a relatively late flight (10am) so it's not as bad as a 6/7am departure.
If anyone wants to offer a counter-point, I'm all ears!