I would just add that it should probably be based on how wide your CP is going to be. I ordered the ultimate widescreen bartop kit from HaRuMaN, which is designed to accommodate a 24" widescreen monitor. I ordered this specific model, however, because I wanted enough room on my CP for 2 player (6 button each) controls and a trackball in the middle. If you're only planning a 1 player or standard 2 player bartop, you probably won't need that wide of a CP and can stick with a 4:3, which is going to display most games better. Mine just has black bars on the sides for most of the older games, which is fine, but not super authentic...
That being said, I love my bartop and wouldn't change a thing