Hey there -- resin.io founder here. Resin is indeed starting with our hosted service, but we have committed to, and are making decent progress towards, building a completely open source platform.

If you need pure OTA, mender is a great solution. Resin is about containers first and foremost, with full-blown host updates being a lot more rare, since the host is a lot simpler.

Different things work for different projects, hope yours goes well!