1st - Davies is a right hander.
2nd - Harrison could get the call as a MR mid-season, but since he's a SP long-term, I can't see him becoming a LH set-up man and he likely starts the season in the minors.
3rd - McBride is effective against LH and so-so (at best) vs. RH. He's not really someone who has shown the ability to be a primary LH set-up guy.
As such, with the predominance of RH set-up candidates (Yates, Paronto, Villareal, Stockman, Boyer, Devine), it is easier to slide Soriano into the closer role and replace him internally than doing the same with MGonz.
Throw in MGonz's elbow questions, his poor performance to date in ST and how Soriano has done, I think Soriano would get first choice.