Picking Faith was easy for me. Faith was already at the buttom. She only had 2 ways to go, death (which she tried via Angel) or up. And once she stopped running and decided to take the downtime in prison (which is usually not the best environment for that) her story has been amazing. She did the adult thing which was facing responsibility and working hard to make herself a better person. And this continues on in the comics.
Willow's personality flaws become glaring by S3. Whenever she feels underappreciated she turns to magic to make herself feel better. When she lashes out at her friends in S4 when Oz leaves and complains her friends aren't there enough and again turns to magic. This trend eventually leads us to S6 where it all turns up to 11. At this point Willow is so power-mad that she resurrects the unwilling, kills deers, mind fraks Tara and a multitude of other things. And since we learn in S7 that magic is not an addiction (lol way to contradict S6 there writers) then Willow's S6 turn comes entirely from Willow's personality flaws. And that was actually the original storyline until AH got the writers to change it. Willow has nothing and no one to blame but herself and her unwillingness to see her own flaws, deal with them and make a change. If she can't better herself then get help. Shrinks exist for a reason.