I can provide you with my thoughts, they are a little one sided but I feel, for good reason. While everybody believes they have the next best idea not everything even great ideas take off. Thus the whole concept of the MVP. Get something out there get people using it, start generating revenues and then invest further. It is my opinion that because the responsively designed website can reach a higher audience, ie everyone with the internet, mobile or otherwise this is the best direction for 90% of MVP's.

The thought here for me, is simple. Build it once. With an App, you will have to build for the given platform(s) iOS as well as Android, unless you wanted to test it on a smaller segment then focus on just one, probably iOS. But then don't forget that this will only work on mobile devices, and because it is an app built for a platform that is constantly changing you too will need to upgrade the app, in some cases just to keep it alive, rather than focusing on enhancements that your user/customer is looking for.

Those are just a few of my thoughts, the other is that building an app will need the involvement of a different developer base and to do it correctly will probably increase your overall costs to get your MVP to market.

In the end, I would say build something that is accessible to a wider reach then once its proven its viability start rolling out the iPhone and android apps. (I am pretty sure Clarity itself is a case in point of website before app development).

Hope this helps.