I think it would be more productive to make the site work well on mobile first. That's probably a lot easier, and more convenient for most users. It also would probably not be that hard to make it work well on both iOS and Android, whereas you'd have to build separate apps for the two platforms.
Also, that way it supports bookmarks, tabs, history, and all that other stuff built into a browser out of the box. Plus, if someone sends you a Smashboards link or links to an external site from within Smashboards, you don't have to switch between the app and the browser (this is annoying when it comes to something like Facebook, but Facebook is a big enough site with a polished enough app that it's okay).