I try to do the cardio first thing in the morning (and I manage it most of the time), but I do the weights after work (before I make dinner). The cardio is only 20 minutes, so I can manage to drag myself out of bed just a little bit earlier, but the weight sessions are more than twice that long so I prefer to do them after work. On those 3 cardio days I like the freedom of not having to do a workout after work -- when I finish work, I'm done for the day.

I do everything at home for now (they gym I joined is still under construction and won't open for another 2 - 4 weeks).

The 2 times I did cardio later in the day (after work), I did notice that I had more energy while I was doing the sprints. That would lead me to believe that the theory that first thing in the morning uses your fat stores is probably true -- I had more energy at the later sessions because I was using more readily available energy from the food I had eaten (even though I made sure I had not eaten for at least 3 hours before the cardio session).

I think, though, each individual needs to do what is best for them. Doing the workout is much more important than what time of day it is done.