Word! Sub4sub helps build your numbers, but not really an audience. Shout outs are certainly going to generate a more legitimate, active audience, because I wouldn't want to associate myself with a channel that I don't admire in some way. And if I like a channel, chances are good that at least some of my followers will like it as well. It's a more authentic exchange.
That said, if I were playing dirty pool I might use sub4sub to get my foot in the door (getting the first 100 subs, maybe). Any more than that and the subscriber-to-activity ratio starts to look unattractive to potential collaborators. IMO.