Will the admin panel be deployed on a separate server? If so, this may be a good reason to use a separate Phoenix app, with shared functionality in the umbrella app.
Will there be possibly multiple frontends with one admin panel managing them all? This may another reason to keep them separate.
As far as a simple web app goes, I would keep them in the same Phoenix application. If you find that later they need to be separated, it should be fairly straightforward if you have designed your modules and interfaces well.
