I have a temperature controller controlling my heaters.
I have my temp probe in the return section of the sump and my heaters in the section immediately after. That way the heaters heat the sump, then the tank before getting back to the temp probe. I did this to make sure I was taking the temp of the water coming from the tank.
With enough water flow, it really shouldn't matter as it's all being churned and warmed at pretty much the same rate.