Davies had a much better Super 15 this year than Turner, so gets Davies gets his chance ahead of Turner. It's not too complex. One thing that has impressed me with Davies this year is that his game has become much more well rounded. His defence and involvements are way better than the previous year, as was his physicality. Some of the tries that Davies scored this year I don't think that Turner would've finished, giving Turner's style of play and form (eg the 2nd? or 3rd? Davies try in the Blues semi, after he took a short ball from A. Fainga'a and still had to beat a couple men to score). This year, I would rate Davies' finishing ability above Turner's.
Originally being a fullback, I think that Davies is now an out and out winger, so I would also agree that Turner has the nod on versatility, if you need it for the squad (Turner makes a better fullback and much much better 13 than Davies, and although I haven't yet seen Turner shine at either position in Super rugby, but he does cover both adequately). On a 5/2 bench split, I can see why Turner would be a better option, depending on the back reserves, especially if the consideration was for either Davies or Turner on the bench.
Turner for me has been disappointing for the past two years - people talk about his highlight reel, which is impressive, but in the last two years he just hasn't live up to his touted potential. I don't know whether it's the players around him, or not, but he is too rarely seen in flight, and too rarely seen busting tackles (although the stats say he does as much as Davies). The one-on-one tackle busting is often what makes Kiwi wingers, and wingers like Mitchell, Ioane, even J'OC, very dangerous on attack. It was also something I thought that Davies didn't do, but has done this year to finish some nice tries. However, it is worth noting that one of Turner's better games for the last two years was vs the All Blacks, where I thought he played very well indeed.
We'll need to wait and see if Davies gets a real chance against a top 10 test team this year, and see how he goes then.