Controlling robots with your eyes, awesome!
Performancewise there is no big difference between the Tobii REX and Tobii EyeX eye trackers. They are both compatible with the low-level Gaze SDK and the low+higher level EyeX SDK and you can performance the same kinds of interactions. The big difference is the price and availability. If you want to start working now, you should order the REX, but if it is ok to wait until March and if you want to save some money I would order the EyeX Controller (right now we also have it on promotion price, see http://www.tobii.com/en/eye-experience/buy/)
I don’t know so much about the good old Tobii X50 since it is a discontinued product, but I wouldn’t recommend to build any interactive applications with it. Your robot deserves better 🙂