To run at night or early morning?  Both are great but different experiences.

Running at night is my favorite for the summer time.  Always cooler temperatures, plus sunset is so late in the day anyways that running at 9 or 10pm doesn't feel like it is so late considering how recent the sun set.  Running at night also can be much more convenient.  Things at the house are done and settled (like kids in bed, work done, house in order), it is quiet and when you get done, the day is done.  This is compared to in the morning having to know that when you get back there is a list of things to take care of and the household to get ready.  Mornings seem more like a time crunch, whereas evening seem like an unwind.  Also running at night seems to be my best speed workouts.  I am more apt to want to really push on intervals or hill climbs, knowing that I will sleep really great when the effort is done.

Running in the morning propels me into the day.  There is something exciting about waking up before everyone else and getting out the door on your own.  Often the stars are still out (depending on how early).  I wouldn't recommend morning runs for speed workouts, but endurance training on long slow paced runs.  The longer and even paced, the better the endorphin kick and a great boost to getting the day started.  Breakfast feels well deserved after a nice run (I usually run on very little in my belly...maybe a small bowl of oatmeal or a banana and a glass of water).

Mixing it up is good and often necessary just to find the time to do the runs, though I wouldn't suggest doing a night running followed by a morning run the next day.