Depends on the size of the job. You could use fast on a hot day for a small part like a mirror cover. You could use slow on a cool day for a complete. Also reducer selection can change things, for example you could do a 1 or 2 panel job on a warm day using fast activator and slow reducer.
So you might get better answers by giving your intended range of use cases, number of panels at a time, ambient conditions, etc.