Sinister Serpent is hand advantage, but Treeborn frog is field advantage. I personally like Treeborn better as it makes tribute monsters more playable. Unfortunately in this era of Monarchs that is a gamebreaking capability.
I agree with Kos. Sinister was good during its time, but the fact that it's banned does little to affect the game anymore. And with so many decks running multiple tribute monsters, as well as Mobius being popular himself (m/t removal for the frog's effect), Treeborn has become a staple in almost any deck that isn't burn related.
I feel they are both equal for the metagame they are involved in. Both have game breaking effects. Treeborn Frog is more so because he hits the field instead of the hand making him better because you can get rid of the sac monster thats been laughing in your face for 4 turns.
i am pretty sure that neither card is better than the other. it all depends upon the situation at hand. both cards offer limitless card advantage. yet the frog offers field advantage for tribute abuse and sinister offers hand advantage for discard abuse. so neither of them really has any hig ground on being superior. they are both superb cards.
both are very powerful..serpant gives you a spare card in your hand every turn and treeborn gives you protection on the field to an extent. one of the decks i run is monarchs and i get to pull one off pretty much every turn just because the frog gets to come back. i would say serpent is better tho imo