My honest view, assuming Jamie Roberts makes the party, is that he has a place as an experienced head on the midweek squad and no more than that.
Reasons:
Hogg is SURELY the full back number one pick. Has to be. And I am not convinced that 1/2p has done enough to even make the plane (a great shame - but that is the way of it as he comes back from his career-threatening injury). To win in New Zealand you absolutely HAVE to kick everything on offer. So Farrell MUST be one of the centres, with Sexton at fly half. You just have to have those kicking options. With Farrell as one centre, I cannot make a case for Roberts being the other. Jonathan Davies, yes, but not Jamie Roberts playing Gatland crash ball all the time. Plus, you would imagine that you would have North drifting in from his wing to offer that, and I would play Watson or Nowell on the other wing to provide the rapier contrast to North's claymore.