Difference between monthly, daily and hourly solar radiation
as you are working on a Solar Tracker, you want to be concerned with the shortest time measurement as the purpose of the tracker is to identify and follow the area of sky with the highest solar radiance (this will always be where the sun is on a clear day). Between your choices, hourly is the shortest for simulation. 

In real life operation, a shorter interval is necessary because panels lose efficiency by a noticeable amount over a 15 minute cycle; and, because you do not want to have micro-adjustments due to short transit cloud formation as that wastes energy in moving the array for lesser results. Your tracker needs to have enough intelligence built in to know if the change in solar radiation is just due to cloud, an obstruction or sunset.

Good luck.