Should I workout on an empty stomach in the morning, or in the evening an hour or so after I eat dinner?

There are really no specific physiological reasons for training in the morning verses training in the evening. It depends on when you feel best, or what your schedule dictates. When, and how much you eat before training is also an individual thing. Let me give you some "gym talk" pro's and con's on these subjects:

Some trainers recommend exercising in the morning without eating anything in hopes that the body will utilyze stored calories (fat) more efficiently during the workout. However, doing this will not create any significant increases in the amount of calories burned and as any good physiologist knows, it's not just burning fat that counts, it's burning more calories that's important. If you are starving and light headed halfway through your workout you simply won't be able to burn as many calories as you would have if you had eaten a little something! Also, don't fall into the trap of thinking that you shouldn't eat anything because then you will just have to work out extra hard to burn those extra calories. A nice, even intake of calories throughout the day is the best way to go! If your workouts are pretty easy, say, 20-30 minutes of cardio when you get up in the morning, then you will probably be ok if you don't feel like eating beforehand. Again, it's somewhat of an individualized thing.

To answer the second part of your question. Some trainers recommend exercising in the evening after dinner because this increases your activity level during a time when you might normally be sitting in front of the TV storing away all those calories! This makes a little more sense physilogically speaking. Exercising after eating your last meal will help to make a storage hormone called insulin more effective. This basically means that your blood sugar levels will be more consistent and you will be less likely to get those dreaded cravings which can push up your daily caloric intake levels.

For me, l like to have something in my stomach before I train in the morning. After sleeping all night my blood sugar levels can be pretty low and I just can't generate enough energy to get a good workout in without eating something. Now, I'm not going to eat a huge breakfast or anything, just enough to give me some energy. I will usually have a meal replacement powder like Myoplex or Met-Rx with some fruit mixed in. Or I might have a small bowl of oatmeal with some vanilla Whey protein mixed in. I usually don't train in the evening because I live in a gym and I can pretty much train when I want to during the day! But that's me. You may be different. Remember, it's more important that you simply eat small, consistent meals throughout the day and exercise when it fits into your schedule best. Don't think that you have to exercise at a certain time or that you should or shouldn't eat beforehand! It's really up to you!