Both actually work in improving the quality of your sleep and whether you should exercise in the morning or in the evening depends on your chronotype. However if you decide to exercise in the evening you should aim to be done with it at least 2 hours before your normal bedtime.

When I work out in the morning I sleep like a rock and can fall asleep as early in the evening as I would like.

When I work out at night I usually feel energized afterwards and tend to stay up later.

I did it in the morning, at 6am. The reason why "did" is because I now go to a swimming pool instead.