MBP, you have to try before you tell. Really.
As I said my qh likes eggbutt, my paint likes loose rings. If your horse is busy in his mouth (and I'd assume he is being OTTB) I'd say loose rings may work better (plus moving rings won't let him "lock" the bit how my paint did with her eggbutt before I switched).
P.S. Personally I don't like D-rings for whatever reason (I like that mouth piece/rings connection in eggbutt more, because it's very smooth). Although I know they are hot in hunter/jumpers around here at the moment.