Black Widow is faster, more agile, more acrobatic, and has an edge in fighting ability, based on feats. Baleman should be stronger, but his strength is not going to mean much if he can barely land a hit. So my vote goes to Natasha.
Just based on his physical appearance in the Dawn of Justice trailer, Batfleck already looks big and buff enough to KO Baleman's ass with one hit. But will wait to see how uber Snyder makes him.