Microsoft provides an important differentiator when it comes to software, and that's the inclusion of Microsoft Office 2013. That includes Word, Excel, PowerPoint and OneNote. There are ways to access these apps from an iPad, of course, but only Microsoft offers local, native access to its popular Office productivity suite for the ARM-based version of Windows. The apps included with iPad are nice, but offer nothing close to the functionality of Microsoft Office.
It's also worth noting that just as iPad will run only apps obtained through Apple's App Store, Surface will run only apps downloaded from Microsoft's Windows Store, which is the exclusive provider of apps for Windows 8 and its Metro-style interface. There's a Desktop Mode, but it won't run third-party apps. Due to the sheer number of people using MS Office, we have to give the software advantage to Surface.