If you go with the ball valves, do not use unplated brass ones as brass, like copper, will react with the beer (which has a far lower PH than water), potentially leaching off flavours and harmful chemicals.
Chrome plated should be ok. Stainless would be better still. Another problem with ball valves is that they are very difficult to sanitise effectively. Most of us get away with using them on our boil kettles because in that situation they get hot, have hot wort flowing through them, and/or are not in contact with vulnerable wort for very long. On an FV it's a different story.
A better alternative might be a plastic tap of the type found on most fermenters. The Temple of the Big Red Hammer sell these for about $1.50.