In general, when trying to evaluate closers, I think it's best to assume that a better pitcher will have more success than a worse pitcher. So, I'd rather have Mota or Ryan than Graves or Percival, simply because they're better pitchers.
Last year, there were two notable new closers at the beginning of the season - Joe Nathan and Arthur Rhodes. Nathan, though young, had shown himself to be a good pitcher, and he was successful as a closer. Rhodes had been a mediocre pitcher his whole career, so it's unsurprising that he was a poor closer. (The only real exception I can think of is LaTroy Hawkins.)