I am asked this question a lot, and it really depends on client goals and when they are at their best for various training purposes. For general purposes, I would say put the energy first towards the goal of the day. If weight training is tops on the list for the day, then do that first and then cardio after. Energy will be best expended that way.
You will get many different opinions on this, so go with what you feel is best for the client that day.