I guess the storyline will be affected from that point onward from my decision, so which one was the best in your opinion? Do you get better mini-movies, boss fights, items, old/new friends, treasure/dress spheres/garment grids, etc. with the Youth League? Or New Yevon? Personally I would pick New Yevon, only because Yuna is already familiar with their shady dealings and would be better equipped to handle their villiany(again). NO SPOILERS PLEASE.
And I love this game much better than FFX. Especially the dresspheres and grids. Much more fun that that horrid sphere grid from FFX.