Siphon has a better balance to it when dealing with larger paint quantities. Gravity is one less step to clean and better for quick color changes. My Iwata gravity fed doesn't even had a cup, just a slot so it only holds a few drops of paint but allows for changing color on the fly and great subtle shading. It would take about a month to paint a 1/16 Tiger with it.

After using siphon for years and gravity for the past one, if I had to have only one, it would be a medium sized gravity fed cup.