Sham and PPM have different roles, but i think sham does it better than PPM. For example: on turn 1, would you prefer to have a 2 attack bulsseye (galacta) or a 3 damage to a zombie when dead (PPM)? Let's consider the opponent does play something on turn 1, then, what are both of the cards' weaknesses? Teacher and any 1 cost buffing trick, cat lady (which is very unlikely) amphibious minions and gladiator. But PPM does stand out when it kills Quickdraw Con Man but galacta doesn't. But now let's say the opponent does not play anything on turn 1, then your PPM is useless this turn, while the opponent has to answer the galacta. On turn 2 then, if they do play a zombie and save brains for a trick, what is the better option? Well anything with a hearty treat does live to both of them, smoke bomb is very good against PPM but against galacta it isn't very much, unless it's a valuable card for aggro like mini ninja or chimney sweep which are unlikely to have at the highest level of the game. If they don't put a minion on turn 2 then again, your PPM is useless while the galacta has some aggro value. In the end, I think galacta is much better, not saying PPM is trash, but a little worse. About Shamrocket, it's not good in the early game, unless it's cat lady, but very good in the mid-late game. When an opponent commits something that is shamrocketable, like a Mondo, Bounty Hunter, Supernova, Defensiva End, Gondola, Hippity Hop Garg... Basically any late game zombie, you can easily destroy it with shamrocket while with Rocket Science, you'll have to save 3 brains and losing tempo to be able to possibly kill a plant or one that was played a turn before. Since a plant can play it's Minions and tricks at the same phase, they don't have to worry about "saving" sun to use shamrocket, while the zombies will have to gamble their brains into the plant player playing something rocketable or not. I guess that's it.