If Gatland goes for Davies and Roberts in midfield, it will almost certainly be 12 Roberts, 13 Davies. Davies has more pace, and that's needed more at outside centre than inside.
I wouldn't have Martin Roberts anywhere neer the squad, he's a gobby shite that doesn't know when to shut up. His service is quite good and he's nippy, but he's not the quality required. I've been more impressed with Knoyle, and I don't think it will be long untill he overtakes Phillips unless Phillips ups his game massively. Knoyle has a better all round skill set, with similar physicallity and agression. He's still a little way off though, but I can see him being Wales' third choice this 6 nations with Rees out. If Phillips doesn't start, I'd want to see Peel back in there. Posts WC I can see Knoyle and Lloyd Williams being our two scrum halves as both are shaping up very well.
I've never seen Priestland at centre, and he won't be considered there for Wales. Hook on the bench over Priestland if he doesn't start, with Byrne there aswell if he's not starting.
I wouldn't be surprised if Hook get's a decent amount of gametime at 10 in the 6 nations. Gatland has a tendancy to do things slightly differently than the regions, and he knows Hook will be playing 10 for whichever club he moves to in the run-up to the WC. We need a quality backup to Stephen Jones and Hook is still that player at this moment in time imo.