They are not totally different at all.
Windows 8 is a super set of windows RT.
Windows 8 is windows RT + desktop mode.
Anything you buy for WinRT will also just work on Win8.
However, that's not correct.
Windows RT runs on ARM processors, and as a result all binary files (programs) have to be compiled to run on ARM.
However, Windows 8 runs on what is called x86 processors, and it's binary files have to be compiled to run on x86.
And that's just the most basic difference, that completely ignores any other software differences.
What you buy for Windows RT will not just work on Windows 8.
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I still have my n900. Gee, what could have been, if they hadn't been such cowards!
As someone still sticking to his N900, I think I'm going to pass on this latest offering.
That said, have you looked at what Jolla, a company of ex-Nokia employees is doing with MeeGo, the successor to the Maemo OS which our N900s runs?
Personally I'm looking forward to what they have to offer.
If you can't understand it, it is intuitively obvious.