I do mine in the morning and that has worked best for me. At night, I seem to be too tired to get into it and I dont put enough effort in. In the morning, I seem to be ready and awake--but, thats me. I arrive at the gym at 0500 and get my stuff done and this has worked well for me for a while.
Tough to say which one is better because we all work different schedules, etc. Try em both and see which one works better for you. Good luck.
-slide