It's all in what you want in a game -- and I mean that not to be non-committal but rather to be honest.
OOTP is more of a total business sim of baseball, you can get mod packs and whatnot to play historical seasons, etc. PureSim has the entire History of Baseball built in and focuses more on the on-field product than OOTP does. Neither game is bad where the other really shines, but they really do have different focuses.
I personally prefer OOTP because I like the business aspect of Baseball and I typically like to skip through micromanaging each game -- but both games are really quite solid.