Howdy Sonex, welcome to the forums
Although I use siphon feed airbrushes, I typically use them with a color cup that is, I believe, 1/4 oz. In my experience, I often use it at least a third to half full, which makes me think the 1/8 oz would be the better choice. If you are intending it to maybe be your main airbrush, I would definitely lean toward the larger cup.
Andy