Godox round heads have built in modelling lights. The V860III’s modelling light is not in the flash head, so its less useful

Godox round heads are compatible with magnetic attachments, but for speedlights, rectangular heads are more versatile IMO. There are tons of accessories for rectangular flash heads and there are flash brackets that a round head won’t fit into

Theoretically a round head with more surface area will create softer light than a rectangular head, but its too small to be a noticeable difference. I wouldn’t recommend using flash without a modifier or bouncing it, so the head shape ends up being irrelevant to the light quality

A round head will provide a nicer looking catchlight, but that only matters if you’re using it without bounce or modifiers