In no to light breezes, there will be little difference if comparing to a soft top that is up. Any top as opposed to no top will be affected more by winds as the wind speed increases, the more 'frontal' area presented to the wind, the bigger the winds effect. As pointed out there will be a larger difference between curtains and no curtains. Also note that if you have equipment mounted or stored on the hardtop ( radar, downriggers, antennas, rafts etc), that will increase the wind effect.
The funny thing about the question is the fact that as the effects of the wind and the hardtop could help docking depending on their direction. Having a slight to moderate breeze pushing you against your dock can be very helpful when docking especially in tight spots.
One other point, a top, hard or soft, will affect speed in most cases. Therefore top ends speed will be slower as compared to the same boat with no top or a Bimini that is down.