[zrgiu_]

Depends on the nature of the startup and your development lifecycle. If you do rapid development, I would go with Android first, iterate very fast based on the feedback from your first batches of users, make decisions based on any usability/feature complaints, and then release an iOS app which has maximized chances to do the best it can on the AppStore.

If your app is highly social, a facebook app would probably be even more appropriate, since the first users don't need to update your app to get the new changes.