In a company I'm working we are planning to get some devices in order to test our software on it. We are doing web app, so I need to know Is there a big difference between Windows 8 IE, Windows phone IE and Windows RT IE In other words should we test on all three devices or testing on desktop IE should be enough?
As far as I know, the Internet Explorer found on Windows Phone uses the same engine as the desktop version of IE. I think the only difference between them is the viewport/screen sizes that they'll be viewed on.
I think really you can test mobile sites on the desktop version of IE the same way you would with Safari, just resize the screen.