shoot at 60 fps and then conform the clip to either 30 or 24 fps depending on what you want to use for a frame rate for your editing timeline. It is important to conform instead of convert because conforming will stretch out the available frames to the new lower frame rate whereas converting will drop frames and still play the clip at normal speed at the new frame rate. So if you shot a 60 fps clip and then conformed to 24 fps, the shot will be perfecting smooth slow motion and will be playing at 40% of the original speed.