Hi John,
There is no official specification sheet for the Tobii REX at the moment, but performance-wise it is similar to the X2-30 with a sample rate at about 30 Hz. The big difference is that the X2 units are made for analytics purposes, delivers a stable sample rate and is compatible with analysis tools like Tobii Studio and Tobii Analytics SDK.
For interaction purposes in consumer computing, the user experience and a consumer-friendly price/form factor/power consumption is more important than a stable sample rate, so we do not believe that having a high, consistent sample rate is super important for eye trackers like Tobii REX and EyeX Controller. But of course it depends on your use case.