Mobile site AND app. Apps and websites generally appeal to different audiences. Because a mobile site is a browser experience it lends itself more to customer acquisition. Apps offer a richer user experience and as such they can be better for customer retention
A good place to start is a review of your Google Analytics. How many visitors to your website are coming through from mobile devices? What about iPhones and Blackberry users? These groups show a high propensity to download buy android app reviews so this may be a good indicator for you. If your budget allows, maybe you can test the water with an app for these users and decide if you want to have a mobile site as well. Either way, a marketing strategy that doesn't consider mobile is no longer becoming an option.