If the requirement includes the ability to run Windows, then Microsoft obviously wins. There are ways to run Windows apps from an iPad, but all rely on network connectivity and remotely controlling an app running elsewhere. Also, Microsoft Office runs natively on Windows and will present a mostly seamless platform for interoperability.
For the enterprise, iPads and other non-Windows devices present a series of security challenges. Devices running Windows 8 Pro can join existing domains with existing credentials and be forced to comply with existing policies. On the flip side, the intuitiveness and stability of iOS and sheer number of available apps make the platform almost irresistible. But unlike iPad, which will only run apps downloaded from Apple's App Store, Surface Pro can run apps "side-loaded" into the Windows 8 "Desktop" or Windows 7 compatibility mode. To run apps in Windows 8's "Metro" mode, they must be downloaded from Windows Store. Despite the enormous iPad app ecosystem, we give the software advantage to Surface Pro.