I started Outcast, Viktoria specifically, then branded out to most of the faction. At the time, the only other masters I liked were Ironsides, The Brewmaster, & McMourning.... but I wanted to have more than one master in a faction, because if you declare a faction with only one option available to you, every one knows what you're playing before you declare Masters, so have at least 2 to pick from.
So it's similar to your first master choice, who do you find cool? Don't think about faction or story or other models that it can hire.... which non-Outcast master just jumps out to you and screams you?
Alternatively, if you're playing Zipp or Jack Daw already, sliding into Bayou or Ressers, respectively, may be easier for you as you already have a foot in the other faction.
And like Morgan said, maybe way until November when Explorer's Society drops, there may be a master coming out that's already calling your name.