Ko is arguably better than Yang now. Actually many of Yang's fellow players have been doing way better than Yang in terms of tournament outcomes.
Yang has the talent to be one of the best in the world indeed, but he lost the desire to play since several years ago, and, as a consequence, has virtually never practiced to prepare himself for tournaments. On the contrary, Ko is a very self-disciplined young player (right now 22) who practices everyday like it's a work in the office. Taiwanese who knows enough about pool will tell you that Yang is about the only player who relies on his sheer talent, not hard work, to play at such a high level. But then he doesn't play at that level anymore.