We have seen Johnson perform on many occasions, but have seen very little of O'Neill, although he comes with a good reputation and was impressive in at least one game.
My own feeling is that O'Neill is the better player in virtually all aspects of a central midfielder's game. If we play with a diamond, I think that he could play behind the striker, where touch and passing are all important. To accommodate him, certainly in away games if we play a single striker, to overcome the problem of width on the left, I would suggest Josh out there. With O'Neill and Howson to supply the wingers and Tettey providing defensive cover, the 4-5-1 could work. And with Wes not far away and Vadis due back in a week or two, it could be worth a trial in the meantime.