Hi,
I understand the confusion, it is not easy to keep track on the different Tobii devices, especially since they are all called something with “Eye” or “X”. As developer, you should only need to be care about the Tobii REX and EyeX Controller. (The PCEye Go and X2-30/60 trackers are products manufactured in smaller volumes that have been adapted to work with the analysis and communication software respectively)
As you say, once EyeX is released we expect a majority of people to purchase EyeX rather than REX – as it comes at a lower price and functions the same way as SteelSeries peripheral to be released later this year.
There are exceptions though, for example if USB 3.0 is not an option, or if you are working on a non-Windows OS (EyeX Controller only works on Windows). There are also some other differences such as EyeX being thinner and wider, but not as high. Also, the EyeX only has one camera in the middle while the Tobii REX has two.
On the algorithm/firmware side, the EyeX Controller has a lot of smart functionality to make use of the less expensive components, so the total performance is about the same. I’m afraid I cannot give you any more details than that. So the short answer of the question of which eye tracker to buy: If you are developing on Windows and have an USB 3.0 port –> Get the EyeX Controller!