Having looked at it a bit more, here are some differences that I noticed:

    - This is written in python. DNSChain is written in CoffeeScript and runs
      on NodeJS.
    - This focuses on a command line utility, whereas with DNSChain you don't
      need one (just do an HTTP request).
    - opendig is like dig, so it doesn't provide your devices with DNS service
      (DNSChain does).
    - DNSChain focuses on security, signing of responses (though that's not
      implemented yet), replacing X509 PKI, unique anti-censorship features
      thanks to partnership with Unblock.us.org project, public key
      distribution for JavaScript apps... See the full list on the GitHub
      page.
    - ONS seems to be working on competing blockchain specifications to
      Namecoin's existing ones. DNSChain doesn't touch that.
    - DNSChain intends to add write access to the blockchain. It's not clear
      whether ONS project does or not.
    - Namecoin and DNSChain predate ONS and are active projects with different
      functionality that ONS seems to be replicating parts of, so I'm not
      sure why the authors chose to not collaborate with either of those
      projects and tread off on their own.