Both "CTRL" and "CONTROL" are common. As a stilistic rule, write (print) a word out whenever possible. If you can't write it out due to lack of space, shorten it. Collapsed abbreviations as "CTRL", where letters inside are left out, are rather ugly, don't you think so? Or do you want to read something like "BCKSPCE"? I would rather prefer to read something like "BACK", "BACKSP." oder even "BS". If practically necessary, as for an 1u for example, "CTRL" is tolerable, though.