Honestly I think they can go either way as far as who might win Wrestlemania if Becky and Ronda wrestle. Eventually someone has to beat Ronda, and Becky is probably the top female performer in the company at the moment. If anyone is going to beat Ronda, it would be her.
Basically I think whichever loses their belt and wins the Rumble will probably win Wrestlemania, because that will justify having them drop the belt. If Becky drops the belt at TLC, I think she'll be the one to win the Rumble, earn the title shot, and go on to win Wrestlemania. Conversey if Ronda drops, it'll be her winning the Rumble and probably winning at WM.