If you're going with the ES-8, it should DEFINITELY be next, as it poses a bit of a learning curve due to the fact that it connects to external software, and you'll have to sort out how to best integrate that with your DAW for the optimal results you're looking for. After that, then I think the Stereomix should be the next pick so that you can familiarize yourself with applying CVs to its various functions, and how to best do that. Like any other mixer, you need to get really cozy with how it behaves before the next steps.