Makes no difference. Water should be moving briskly enough that the entire tank volume is pretty much the same temperature.
Dont put a heater too close to your return pump, or any pump for that matter, I had an issue with my heater heating up the return pump and breaking the pump. I moved it to my skimmer section and all was good again.
I would put both in the drain section. Iwouldnt want a malfunction and heater melting my skimmer