Both Apple and Google app stores have over one million applications available. These grandiose numbers mean many apps or their equivalents are available for both iOS and Android. For example, Microsoft Office 365, Microsoft Remote Desktop, Cisco WebEx, Salesforce and Bloomberg are all available for both operating systems.
Not all apps are created equal though - the iOS version of Documents To Go office suite can open files stored on Google Drive, Box, Dropbox, SugarSync and iCloud. The Android version on the other hand is limited to just Google Drive.
Some apps are exclusive to iOS or at least dont have direct counterparts on Android. SAP CRM Sales, Oracle Fusion Mobile Sales, Fantastical and Roambi Business Intelligence are a few examples.