Hi Andrew,
At the moment the Tobii REX eye tracker handles large screens better than the EyeX Controller. One reason is that the REX has two cameras while the EyeX Controller only has one in the middle. (that is also one of the reasons why the EyeX Controller does not cost as much).
However, the EyeX Controller algorithms/firmware is still under heavy development and the goal is to be able to handle at least 27″ screens in a near future. But if you need large screen support right now, the REX is recommended (even though I don’t think it will work so well in the corners on the 30″ screen.)