iOS is not OS X in the sense most people mean in; the fact that there is a second sense of OS X (the Darwin kernel, etc) which runs behind OS X is an irrelevance, and is rather like saying "Android is Linux" -- well, if you're an embedded Linux developer looking to modify the Android OS, that's a useful truth... for pretty much anyone else, it's a factoid that happens to be true, but not in anyway useful.

To most people, what OS something runs is the combination of the UI (almost completely different in both cases, even if it's something that runs on TOP of the underlying OS kernel and libraries, which might or might not be hte same) and even more importantly, the software it can run. And in that case, Android, iOS, OS X, and Linux are all almost entirely unrelated.

"Gadget" is in the eye of the beholder. :

The opposite: while RT still has a desktop mode, you pretty much can't get software for it. RT only takes software compiled specifically for RT, which is only available via Microsoft's store, which only accepts "modern UI" software (at this point, at least.)

They're both full x86 PCs (and could be reformatted with Win 7, Linux, whatever you want, although they'd be pretty awkward with anything but Windows 8 or maybe the touch flavors of Ububtu.)

Don't discount ecosystem lock-in.