We are using an ARM-based SoC from NXP. We chose this to ensure that the device can only run signed, trusted code by implementing secure boot and signature verification of software updates. We will make a developer program available in the future.
I didn't use Protonet so I shouldn't speculate about what's similar or different about the products. I think we are in a time right now where people have a strong desire to own their data and are looking for a viable alternative to the cloud. They key, beyond building something very private and secure, is in nailing the experience and we're excited to share more about that.