If you have to pick one, in most cases it'd be web. Mobile devices have web browsers but web devices don't have mobile browsers.
9 times out of 10, if your application doesn't have any advantages on a mobile device (location services, camera services, attachments like square) then there is no point of releasing an app at all- just make a mobile-in-mind web site.
We chose to do web first even though our app will mostly be used on mobile devices. One of the main reasons we chose to implement the web application first was to get a working MVP. I am much more experienced with web apps and have designed our product with mobile in mind. It's currently the best of both worlds when it comes to time and functionality. In the near future we plan on creating device specific apps.
I know this scenario is specific to our case, but I believe many other startups are in the same boat. When you have to choose between creating a two device apps (iOS and Android) vs one centralized web app for an MVP, I'd chose the web app. Again keep in mind not all scenarios fit this mold.
I should have clarified that I wasn't looking for advice for a specific startup, but rather to draft up a general guide of pros and cons to consider for any startup facing this decision.
I'll absolutely take all of the comments made on this thread into consideration when I write the post!
The web should be the first move because it is an easier pivot point for starting. With a website, you can give people a way to see your product and/or interact with it. It also centralizes your product under a findable URL. A website can also get many more users than one mobile platform can. Mobile discovery is lacking so as of now the web is the best way to start your project.
I would start with "web" first. If you hit it off, then build a mobile version. My gut is always to google something on a computer and not to go to the app store and search for apps. But it could just be me.
9 times out of 10, if your application doesn't have any advantages on a mobile device (location services, camera services, attachments like square) then there is no point of releasing an app at all- just make a mobile-in-mind web site.