My last startup we went mobile first and it sucked. We were bootstrapping so also contracting. The pressure of having to maintain two codebases (in this case iOS and Rails) as well as keep clients happy left a particularly bitter taste. That's before you do any marketing too!!
For my new one I've purposefully chosen something which can be built completely in rails and still be useful. So much easier when dev resource (i.e. me) is constrained.