They are both built in the same way, except for the one that has the little "keys" on it.
As for which would be better, it depends on the horse. I know that my Otis is a very hard mouthed and tight in his jaw (from past training that was not very nice), but he's also very tactile and needs constant stimulation. So he'd probably enjoy having a mouthpiece like that because he can play, and it also encourages him to soften without as much bother from a rider's hands.
But other horses may be bothered by it. As I said it depends on the horse.