"My team and I at Parse have worked hard to create a platform that lets..."
I might just start out with Parse if it is so easy to convert it the other way as well.
Edit: Just re-read, I think you meant to say between Rails and Parse -- with Parse, you'd still need Backbone.
A) if you don't use Parse you're going to have to write server code.
B) If you do use Parse and decide to change in the future you're going to have to write server code.
The outcome is the same however with option B you will have a working app 70% faster. Parse is great for moving fast while you search for product/market fit.
UPDATE: Nevermind. I just found the other article on HN that explains how Parse is a storage PaaS. This article makes more sense now.