The problem with 15 is, IMO, there's entirely too many players in the database, especially if we start out with less teams than there currently are in MLB. This means that it would have to be a fictional league if you're going to use 15. That's too much like Porter's league for my taste.
I've been the biggest proponent of using OOTP 15, but I think I would rather have another 6.5 league.