One of the authors here. Deis is better at the moment, supporting clusters, as well as being coreOS based (which is one of our goals as well). Back in october/november, when the project was started, we were ahead for some time, but I ran out of time to work on it. So recently, I found more time and ended up open-sourcing it, hoping to get more contributors onboard.
Yes, we technically could collaborate with Deis, though they a different programming language (python). I also have a few concerns doing major contributions, since Deis developers' company basically makes money off of Deis via consulting -- I'd probably prefer to join the Flynn camp (https://flynn.io/).
Peas seems to be less feature complete. We use Heroku's logplex for storing the logs (something peas doesn't do yet, they do have a development branch that uses mongodb for that), and we also store the github repositories on the server, like Heroku does, while peas requires you to use a separate repository. We also work as a multi-tenant system (although admittedly, accounts need to be pre-seeded right now), while peas has no notion of users. I like tombh's work though, I just wanted to release my own system, since it predates peas.
tl;dr: Peas offers a few less features, but is more lightweight.
