There are two issues here that might make the Surface Pro more appealing as a business tablet.
The first is that it integrates with Active Directory and other enterprise systems. There's no need for additional tools, which can incur per-device licensing fees or extra processes. There is also no need to recreate user or device groups and policies. The typical Windows policies already place can easily extend to a tablet with little extra effort. This reduces overhead, simplifies troubleshooting and doesn't require additional IT training.
The second is that iOS management options are nowhere near as granular as Active Directory group policies, which can restrict and configure almost every aspect of PC operation based on a range of factors -- the particular device being used; the user or group membership; or when/where a person logs into a device. Although the policy options available in iOS have grown substantially over the past five years and can be applied based on a similar range of factors, they still don't offer the level of specificity of Active Directory.
How significant these issues are in a company's decision between an iPad Pro and a Surface Pro will vary. If an organization already has a heavy investment in iOS devices/device management, supporting the iPad Pro may not be particularly onerous, though concerns about per-device licensing of a management solution may still be an issue.
It's also worth noting that iOS can natively interact with some common enterprise systems, the biggest example being Microsoft Exchange.
What about enterprise apps?
While enterprise integration is a point in favor of the Surface Pro, the ability to run corporate apps is a much bigger one. Almost all mid-to-large size organizations have custom in-house apps. These can accomplish anything from time and attendance tasks to inventory control to customer service to filing mileage and expense reports to storing clinical data. The larger and older the organization, the more enterprise apps are likely to be in use. In many cases, these apps are years or even decades old and have been tweaked, patched and adjusted since they were created. In almost every case, they're designed for use with a mouse and keyboard.
Running a full version of Windows means that the Surface Pro can generally run these apps with little or no modification. Ideally, they might benefit from tweaks to add touch-friendly input options, but they'll run natively on the device.
The same cannot be said for the iPad Pro (or any iPad for that matter). There are work-arounds that can push a desktop app to a mobile device (or to non-Windows platforms) like VDI (virtual desktop infrastructure) and DaaS (desktop as a service) offerings, but they add their own challenges. They require an active network connection that's reliable and robust. And they require spending on solutions and infrastructure, which can mount up quickly.
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