Pros: Often touted as the best time to work out, a dawn exercise session is great for so many reasons. By exercising early you leave your schedule open for other plans, feel energized the rest of the day, and are more likely to keep an exercise habit.
Cons: If you're not a morning person, staying motivated to wake up in time to work out can be hard (and lead to slacking off).
Pros: It can combat an afternoon slump, and not only that, exercising in the afternoon may be the best time to do so, energy level wise: your body will have a faster reaction time, muscle strength, and cardio efficiency around late afternoon.
Cons: If you work all day, getting in a midday workout can be a little harder to do, and you may have to cut your workouts short to factor in changing and shower time into your lunch break.
Pros: You can spend longer on your actual exercise time since you won't have work hanging over your head. You can also fuel up and hydrate well before your workout (which may be harder to do if you exercise early in the morning).
Cons: No spontaneous social plans for you if you want to stick to your gym routine, and you also may be finding yourself combating the feeling of wanting to skip a workout in favor of an evening on the couch.