Personally? I'd go with neither. Saul is best off staffbotting, (Not like promotion hurts him, though) and Deke is basically Marcus 2, and is mostly valuable if he is earlypromoted in Ch. 8 in HM. Do whatever you like, though.
Also, how are you low on cash, actually? FE6 throws a lot of money at you. Did you miss the chests in Ch. 12?