Based on recent history at the event....Maria.
Based on recent clay form.... Serena.
If Serena can break the "not past the quarters in a decade" slump she could win the thing. But of the last 4 times she has played she's been out twice before the 2nd week. Maybe her recent red clay wins will spur her...or maybe she will bomb out again. Neither would surprise me in the least.
I ended up voting for Maria mainly because of all the top 4 seeds she is the only one to have made multiple SFs here in recent years...in fact the only one to get that far at all. Unless she is legitimately sick (which I doubt) I don't see Maria bombing out so it will be up to Serena to get to her. I just don't feel confident she will.