I have an ASUS Transformer Prime (Android 4.2.1), iPad 4th Genno (iOS 7, currently ownership of my mother), Surface RT (Windows RT 8.1), and Surface Pro (Windows 8.1 Pro).
Android on a Tablet is nothing more than terrible.
Android on a Phone is Laggy, but it runs fine on them, but on a Tablet, it's way to Laggy, lacks Tablet-optimised Apps, and loses support really quick. But at least it's cheap.
iOS is okay overall, but many sites are inaccessible on it, and like with Android, it's meant for home use only, I can't do my jobs on neither of them. A plus is, if you're a Developer, you don't need to worry about 9283928573 Devices being different.
Windows RT is a great Tablet OS, but lacks many Apps. Disallowing 'legacy' Apps is a bad idea, in my opinion, XDA-Developers have proven Windows RT can run 'legacy' Apps perfectly fine, when re-compiled for ARM. Besides, that's why people don't see an added value to it over Android and iOS.
I'm using Windows RT for home use.
Windows 8(.1) Pro is the best Tablet OS so far. It feels complete, it is complete. The Surface Pro is the only Tablet on the Market I can use for work. After a while, I've got problems with Touch Recognition, so I disabled the Touch Screen Driver, and continued to use the Stylus instead. Also, the Surface Pro gets burning hot after a short while, I've found out that if I use a fan behind that thing, it stays cool enough to at least use it as a Laptop.
I've recently hooked it on my TV via HDMI (via Mini-Display), plugged in a USB Hub with a Mouse, LAN Adapter, and External HDD mounted, and a Bluetooth Keyboard attached to it, and now I even use it as my Desktop PC.
As for Surface (Pro) 2, I'm not sure I want one, I'm perfectly fine with my Surface Pro.