The consensus is that there is no best time to workout, but rather that the best time is the one which will allow you to be committed to your training program. The reason for this is that exercising consistently gives long term benefits which outweigh the difference between a morning and evening workout.
Your preference for morning versus evening training will depend in part on your body's circadian rhythm, which is something like your body's own metabolic clock. If you feel lethargic and out of energy when you wake up then you probably won't want to go on a 10K run in the middle of winter. To answer your question, if you feel exhausted after leaving the office, then your body may be telling you to train in the morning. Perhaps you can find time during lunch for a workout?
There are some differences between a morning and evening workout. Working out hard late in the evening can cause insomnia. Normally in the late evening your brain increases its production of melatonin, and your body's metabolism slows down. This is all supposed to get you ready to sleep, but a hard workout can prevent this. I can personally attest that late night workouts can cause big insomnia. And I have paid the price for this many times, as I often had to wake up early the next morning to train again.
There is an excellent article on BodyBuilding.com which compares many exercise parameters during morning, noon, and evening.
American Heart Association