Well, comparing the numbers, against RHPs, they have similar HR rates but Freeman has the better AVG and better RBI rate so far. Also, the Braves have a much higher OBP than the Nationals do so Freeman would have better RBI chances. So, I'd choose Freeman.
P.S. If there's another 1B that has a better average against RHP and LHP than the average Freeman has against LHP available, pick up the guy too so that you could put him in when Braves are against a LHP.