Base your decision on which one to play first by looking at your opponents deck. If it is something like gheist/roots then play phyllis first because her sob will get her ability through.
In other situations go with oxen since he has the higher minimum and keeps phyllis relevent to the match as she will still get some damage in later on if it is required.
Not so high minimum. And if you have in your hand any other of this 4 cards: Oshitsune, Pan, Dorian or Nellie, the "high" minimum won't have any relevance. And you wil have lots of possibilities of bluffing or not with your starters depending on your opponent's hand. I consider that putting Oxen instead of Rubie is not a bad chance. Try the 2 options and choose.