That is the tough part about junglers. It matters what you want to accomplish in the game. Twitch can climb just because he can carry a team. Sej won't carry but can enable your whole team to dominate which is a similar role to Rammus just done with different mechanics. If your team already has some strong carries in it, then Sej is a great combo, if your team lacks a strong carry then twitch is good. It is and should never be about a certain champ being able to carry you up ranks. It is your play and champ select vs other teams and team comp that will make the difference. Twitch would probably do the best, but only because as a solo player he can carry a team, where when you play solo sej or rammus or many others, you have to rely on your team working together, which is never a guarantee.