Personally from what I've seen, Dawson isn't better than Santi.Quarterfinal of the FA Cup isn´t the right timing to give someone playingtime just because of age.
Best team plays for me,and this means Daws..who is the regular starter ahead of Santiago this season anyways.
I´m not for playing anyone worse over anyone better just because of age under any circumstances.
Best eleven players should play..and if someone is beyond 50 then so be it..way it goes it will happen sooner or later since there is more and more players still upholding their qualities beyond 35.