Is there a functional difference between a ball carb cap and a flat carb cap if they both have the directional spout on the underside?

This really depends on your banger, but the ball cap will give you a little bit more wiggle room (literally) so you can direct the airflow over the entire bottom of the banger. Flat caps are pretty much stuck at the angle of the spout if correctly placed flush on top of your banger, however, it really all comes down to preference and the style of banger you are using.