It depends on the aspect of the screen that you are projecting on! If your projecting on a screen that is Scope, format for that, if your projecting in a mainly FLAT environment go with that container. Whichever makes your image larger to the audience.
I've used OpenDCP in the past and it is effective, but I actually recommend DCP-O-Matic, simple setup and you get a preview of the project. It's free just like OpenDCP, and accepts Prores and DNxHD files then does the full conversion.