If you want a tablet that can be used for development, you'll want one with Windows 8. Windows RT will not allow you to install anything that doesn't come from the Windows Store, except through using remote debugging. So you won't be able to install Construct 2 or Visual Studio or anything like that.
If you want one for testing or just general use, Windows RT is fine. It's basically a limited version of Windows 8 that is made specifically for tablets, while the full version works great on tablets, laptops, and desktops.
As far as which specific one to buy? I have no idea. I've only gotten to know the older tablets, not the ones specifically made with Windows 8 in mind. Just know that the RT tablets are going to be cheaper, since they're made with lower power and less expensive hardware.
I don't think WebGL works in Windows 8 right now, unfortunately. Don't know if that will change in the future. Awesomium exports to an .exe file, so that won't work on RT, because, as I said, it will only allow you to run programs installed off of the Windows Store.