Appel has more risk but more upside than Fisher.
I think they want to get Appel back to his college 2 seamer when he was dominant, Houston had him focus on a 4 seamer which added velocity but was more hittable (see Asher). "With Houston, Appel threw a four seam fastball almost exclusively, and he got almost no natural movement on the pitch. In college he threw a two seam fastball almost exclusively, which added some movement to his arsenal. " He throws the 2 seamer in the low 90s, which would then make a high 4 seamer his show-me pitcher, not his mainstay. The injury last year grounded those plans.
If Appel can spot a 92-94 two seamer knee high and outside, the "money" spots, he can return to #1 starter form. He has the stamina (assuming the elbow thing is a one off), the secondary pitches, just needs a FB he trusts.
With Cozens, Pullin, Quinn, Williams, and Goeddel probably in Lehigh this year, we have OF prospects, what we need are ML OFs. Moniak, Ortiz, Randolph in A ball, what I'd be looking for in a prospect for prospect trade is an OF for Reading.