I have tried both, marbles he likes the "shiny" factor and the pingpong he likes when I prod it about for him. He does not nudge but likes to wrap himself on it. It is still cute
some bettas do enjoy rolling marbles around. and they seem to be fascinated my the shininess and color of marbles (if they cant push them around). theyd be fun for a betta. ping pong balls also are fun for them.
cheers
My fish have never even taken notice to the marbles or ping pong balls I supplied them with. They have however taken interest in marbles if I roll them around outside of their tank. The same with tossing around balls of foil My roommate and I played fishy in the middle with a foil ball around her betta and he seemed to enjoy it