One correction: the brightness keys don't work with the X220 with 10.1. At least not by default.
The X220 has been supplanted by, first, the X230 and now the X240. If you don't mind slightly older hardware, a used X220 has advantages (in my opinion) over the newer models. The big two for me are the keyboard layout and the separate trackpad buttons. I disable the trackpad and use the pointing stick. Without the discrete buttons, I'm not sure if one can disable the trackpad and still use the buttons.
Sorry, I'm not sure about the X1 Carbon.