The problem with mobile first is that you might not fully "brainstorm" all the possibilities. Your mind is so focused on creating a minimal subset that you might miss some good ideas. As you design for larger displays you then realize the ideas you missed earlier. In the desktop first approach you create a fuller set of possibilities from the very start and get a "big picture" view of the app. I think it's useful to create a prototype of the full version to get all the ideas out, and then go ahead and do mobile-first with that clearer understanding.

So, at this point, I've decided how to proceed: Mobile first! But not designing, just thinking, then you'll do responsive.

1) Plan larger

2) Prototype mobile

3) Design larger

4) Think mobile first

5) So responsive

6) Design mobile