I'm afraid I cannot tell you whether rock crabs or experiments are better xp, however, I can tell you that experiments are really REALLY boring.... I find Armored Zombies to be the best training xp in the game. (Way Controversial, might I add.) Though, when I train on them, I use Salve ammy (e), Piety, and a super set. I'm sure our stats are different as well. When you count all these things in, everyone's experience will be different. So, I really can't tell you what to do, but I hope I've helped you. =)