One major negative to coaches (especially if you have a lot of minors) is constantly forgetting that their contracts run out. Not only that, but the sheer number of coaches is a little much for me. While it's realistic, I've never seen positive effects from good coaches in the minor leagues. I might be wrong of course
I've never paid attention to age/experience really. To be honest, I just pick random guys from the coach list that fit my organization and will actually sign with me.
I always play with scouts. I like hidden players, I like the extra "fog of war".