Always use 3 phase when available. Rotating equipment runs more efficiently on 3P, inductive loads like heat panels not so much of a savings. A Watt is a Watt is a Watt so 5000w single or three phase. Another benefit is lower amperage usually meaning smaller wire. 7.5 hp motor single phase 35a 10 ga wire, 3p maybe 18a 12 ga wire.