I'm not sure what the value of a manned mission to Mars would be, really. Sending up robots seems easier and much, much cheaper. Perhaps one could think of it as a step in a plan to terraform Mars, a planet with about 11% the mass of earth which has no electromagnetic field and can't hold on to an atmosphere .. but those plans can probably wait a presidential period or two. Stemcell research can benefit us in so many ways right here and now.
edit :
Sorry for the random reply .. This is about lunar missions of course
Manned missions to the moon is not something I would prioritize over stemcell research I guess is what I mean to say.