I just find most bullpen guys replaceable, thus their value is low. Unless you profile to be at least a set up guy, your value is generally low because you should be readily replaceable by a good team. Bench guys are generally the same way. If that is your ceiling.... then I am going to lean toward the guy with a ceiling that is that of a starter even if they haven't proven it yet because teams should easily be able to find a bench player every year on the cheap.
If bench players and bullpenners were "easily replaceable," every team in the bigs would have benches and bullpens full of minimum salaried players. The fact that teams have lots to choose from to fill these positions does not mean that all of these players are alike.
A player who offers the big league club something is worth more than a player who offers nothing. And, in my estimation, the rankings here and on your site tend to ignore players who are more likely to offer modest contributions and depth in favor of players who still hold a fairly strong likelihood of flaming out completely.