Since Microsoft made Office available for the iPad, a big advantage to the Surface line was eliminated. Since I already had an Office 365 account, enabling all of the extra features such as revision marking that I need was automatic. So I can get real work done on an iPad, though it works best with an external keyboard. The Surface 3 works better in this mode, but not a lot better until it is time to print. Most of my work doesn't involve printing, so that's not much of a concern. Outlook is a totally different story. It only works properly linked to a Google Apps account in an enterprise context on the PC platform. I don't use it at all on any Apple device. This is the biggest advantage of the Surface.
onlysublime's reply made a very good point. No device does it all. There are work-arounds that enable the user to get by. That said, my first Surface 2 purchase was prompted by a really lousy experience drafting legal language using the Office-like apps then available as a work-around. I was convinced never to travel only with an iPad. Availability of MS Office has corrected the major problem.
Incidentally, any AirPrint capable printer (i.e. most HPs and Brothers) on the same network will print from an iPad. Other printers have apps for the iPad, though the experience is far less satisfying. The Surface line just work like a computer. Install the printer and it prints.
Similarly, you can tether the Surface to an iPhone using wifi or Bluetooth, provided that the phone is turned on and the screen is showing the "hotspot" page. A few extra steps is not a deal-breaker, but avoiding them is a welcome luxury. The more secure method - tethering the iPhone to the computer via USB can be done on either a Mac or a PC, provided the PC has iTunes installed. Philosophical issues aside, this is at least possible on the Surface 3 whereas it was impossible on the Surface 2. But for secure communications in a public setting, nothing beats Instant Hotspot. It enables me to communicate with the "mother ship" in time intervals that would be impossible using any of the work-arounds. Pull out the iPad without taking the phone out of my pocket. Select "jbs's iPhone" from the wifi list, and go. Send the document, close the case cover and I'm done. That experience can't be duplicated on any other platform, and when things are extremely time-sensitive, it makes a real difference.
Our business has a conference room AV setup that includes Apple TV. I can display content from any Apple device during meetings. AirParrot 2 enables a Windows device, including the Surface 3, to do the same thing.
My point is that lock-in is much less of a problem than it used to be, provided that the user is open to using work-arounds. It finally comes down to two simple questions: (1) whether you have access to power, either through a wall outlet or a backup battery, and (2) whether you are locked into an application that only runs properly on one platform.
The answer for me to the above two questions is (1) it depends, and (2) no. So I travel with the iPad and the Surface. Data stays in OneDrive, which is accessible from either. If I had to chose only one, I'm not sure what I'd choose.