depends on the market, so it's IMPOSSIBLE to advise the correct approach/choice in this question. However, I'd lean towards a desktop/web app for anything that is trying to accomplish 'platform' level mechanics.... most noobs will use the term platform and app interchangeably but there is a huge difference: platforms are designed for OTHER COMPANIES/brands/power-users to BUILD FUNCTIONALITY on top of/extend the use for their specific purposes... TWITTER is a platform. "an app to buy/sell stuff in X new way or Y speciality or Z local/geographic market" is just an app that is trying to say it's cool by calling itself a platform because there's type major types of uses & 'transactions' involved.... a CHATROOM is an app... TWITTER is a platform. Hope that helps.