As someone who has been an avid fitness enthusiast for about seven years or so, I have to say I've seen the best workout results from my mornings.
You get up, have a cup of coffee and a container of water, then start warming up your body. Have a nice hour-long workout, preferably cardio for me, and your brain chemistry is golden for the remainder of the day.
If you wait till the afternoon, you might end up getting tired, or still feel full from lunch. You could end up getting a call from a buddy for beer, or your wife might make an early supper. Or you might have to stay late at the office.
There are too many factors for afternoon workouts. Best to get it done before daylight!
Especially if you want to lose some weight, it is better to exercise in the afternoon after you have had your breakfast and lunch. You may exercise in the morning then eat after the work out; that won't help. Exercising in the afternoon can also be a way to relieve your brain and muscles from tension caused by stresses of the day. It also helps you to digest well all that you'd have eaten during the day and prepares your stomach for proper digestion of your supper for better sleep.
In the morning when the body is really sleepy and is not fully awake yet, isn't it when you exercise your muscles will still be in the process of waking up and not growing? In the afternoon when your body is awake and not drowsy is when your muscles get into the process of focusing on building or strength/conditioning?