Neither are necessarily harder than the other, It depends on the grade of the route you are climbing and your skills as a climber. You could climb a 5A top rope route that would be equally easy as a 5A lead route. You could also climb an 8A top rope route, and it would be just as hard as a lead route.
One element that could make lead climbing harder than top rope is the fact that you have to clip your rope into quickdraws while climbing. To do this, you have to be efficient at clipping, and you have to be efficient at resting, since clipping, although it may take a fraction of a second, can tire the climber out if they are not efficient at clipping or if the route is just very difficult.
On the other hand, many people who top rope and auto belay may find it a little bit tricky to make their way around the rope that is always in front of them, which sometimes could be a hassle, especially if you are on a very difficult route and the rope is in your way, and you have to move it, or yourself, to get to the next hold. This, like with lead climbing, requires technique and skill.
All forms of climbing have their difficulties, there is not necessarily a harder climbing form than another. Many factors come into play with this question, and it depends greatly on the climbers themselves.
