It depends. Do you want to do it just once and forget it? If so, then use the pad. If you use grease (any of them except perhaps for Shin Etsu - but it is also the hardest of the greases to apply), it should be redone at least annually. Most of the ATF denizens don't leave things alone long enough for the 'pump-out' phenomenon to occur.
. Pads are called "Phase Change Thermal Interface Material" which means they must change from a solid to a liquid (preferably just once or they could be pumped out too) to work properly. The one you have sounds like Shin Etsu PC material (usually gray) which has a PC point of around 50 deg C (the pink ones are Chomerics and need 60 deg C or so to melt). So you need to allow your CPU to rise into the mid to upper 50s for a few seconds to be sure it melts. This may require that you disconnect or reduce the speed of your CPU fan for a brief period depending on the efficiency of your HSF.
. Done properly, there won't be but a degree or two difference between the pad and the grease, so it's better to make your decision on other considerations. If you want further details on how to do the melt, PM me.
.bh.