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Keep the mulch, skip the stone. Mulch will continue to feed the soil as you renew it. Stone will become a real PITA as it collects and becomes partially buried by plant material--leaves, twigs, fallen flowers, etc. And it will reflect a lot of heat in your zone. Stick with the mulch.

Please listen to laceyvail: stone works well on the west coast but it's a mistake around here. Laying a couple inches of fresh mulch each autumn beats the heck out of weeding through gravel or river rock.

Mophead hydrangeas in full NC sun is a poor choice, they need shade in your zone. Look at Bobo instead.