Oxide was involved with Tock before building Hubris. They are similar in some ways with somewhat different "visions" for the end use case. Hubris compiles all "applications" into the "kernel", and relies exclusively on the type system for isolation---in this sense it is a much more traditional RTOS, just written in Rust (and well designed!). Whereas Tock targets applications that are not just in Rust, and may be dynamically load/replaced/removed separately from the kernel---in this sense it is much more similar to a traditional desktop/server OS but designed for significantly lower resourced settings. This also makes Tock more robust to applications that are actually untrusted or unreliable.
There is also of course a difference in development and evolution. Hubris is developed by Oxide and released open source, while Tock is more community based and, thus, more open to supporting a variety of use cases. This is both bad and good. If your use case is exactly Oxide's use case, it's likely Hubris is better than Tock (there is just no design decision baggage for other use cases). If you're use case is a bit different, Tock starts to be more appropriate.
A side note that I think the Hubris-Tock relationship is a real positive case for open source development. Oxide was very transparent and forthcoming when they decided to switch away and offered a lot of useful feedback. I hope they took some ideas from Tock and I think we took some ideas from following Hubris.