What are the differences against Resin.io?
Mender is basically full image update solution while resin is container based. Mender is fully open source, both client and server, resin is having only client open source. Mender is more lightweight, it provides a thin layer to be integrated with the already existing stack, while resin is providing full stack you need to use to be able to incorporate update mechanism.
Mender is also on a less restrictive software license
According to the article Mender releases under Apache 2.0, and all resin.io's open source code is also on Apache 2.0, so it should be the same permissive setup.
(Source: working at resin.io)