the default setup on my ASHP was return temp too, but "return" in this case is a wandering probe that could instead be in a buffer tank. I've now re-set it to flow to try. Power consumption seems very similar in both modes.
In both cases my external probes tell me that return temp pretty much matches the circulating temp in the slab, but the delta and pump speed control are radically different. Tis velly interresting. I'm waiting for a flow-setter coming so I can get the pump speed best optimised. It's PWM but the limits are adjustable, plus I've got a secondary in the plant room too.