Personally, the main thing for me is that I like body-to-body contact, and I find I get more of that when receiving from a harness than from a hand. I don't know it if is necessarily that way, but my experience is that I get more of what I am looking for when a harness is being used.
Even if I am receiving while bent over or on my hands and knees, I feel her thighs and hips as a delicious contrast to the dildo when a harness is being used. But if it is being hand controlled, I find that she is generally sitting a bit away and I tend to just get the contact of her other hand and the dildo.