PBR is a good indicator of player value. The "average" starter with 180 innings should help most teams more than a "better" reliever with 60-70 innings. Generally.

You also need to look at your team. If you have several starting pitchers beyond what you'll need, perhaps you make the deal for the lower PBR closer who can help you more than a long reliever. In this case, the closer is more valuable to your team than his PBR would otherwise indicate.

Just an opinion, but since I haven't won a perp league...