Is a hard top heavier than a convertible?

Not Usually - Convertibles are usually heavier because they require additional chassis re-enforcement (which adds weight) to maintain the structural integrity of the vehicle that a roof would have provided.

In other Words, the roof of a car provides enforcement against the twisting force of the motor. Then you remove the roof, you have to the re-enforce the frame or else the car might actually bend from the twisting force (torsion).