Preacher Curls are a better exercise for the biceps than hammer curls in my opinion. Like Stu said, the main muscle working is different. The bicep works best when supinated, and preacher curls are great for isolating the bicep. You can't cheat on these, because you got you arm supported, thus eliminating most of the shoulder work, and the torso swinging. The preacher curls will mainly develop your bicep, both heads. I don't think you can overemphasize longer or shorter head. Mainly because the originate from the same bone (With few inches of difference) and insert on the same tendon.
For serratus anterior, I've also found push up plus/calpping push up or similar to be the best exercise. To almost work solely on the Serratus, exrx has a nice scapular abduction exercise HERE