Running (or working out) starts the body burning fat after about 20 to 30 mins, and that lasts for some time after your workout ends (not sure how long, someone here might).
Most runners I know have the opinion that getting your metabolism going in the morning helps weight loss by burning extra calories during the early part of the day.
I don't know all of the details on it, but that is the jist of what I have read. I'm sure you can find something on it at www.runnersworld.com
They also have a free training log that I highly recommend using.
Secondly, there is the heat/sun preference. Some don't like the heat/sun, others (like me) prefer to run in the heat/sun and sweat it out as much as possible.
Third, depending on where you live and your route, you might schedule around traffic. I like running along the river in Cincinnati, but during busy traffic the exhaust fumes and traffic make it zero fun. If I get a chance to do those routes at off times, I usually go for it.
Fourth, scenery availability. The routes I run typically have other runners out...and during peak hours in the summer, all the runner chicks are out and that can motivate you to keep on going.
Hope that helps.