Good question. Captain A has the super-soldier serum that gives him peak human abilities, but USAgent is a metahuman in strength terms. Both are good fighters, with Cap having more extensive experience and USAgent being a tad bit more brutal.
I want to say Cap, but i wouldn't be surprised if it went the other way.
I agree with spetnaz. People on here always shoot down John Walker when he's compared to Cap, when in fact he has a big advantage over him, that being his superior class 10 strength and his training from Taskmaster. It can go either way, but the scales usually tip in U.S. Agent's favor due to his immediate advantages.