Mitchell has had a quiet couple of weeks, but he has been outstanding this season.
If we are considering only the players regularly playing 13, then there are really only 4 to choose from (one for each Oz team). Chambers had a good game against the Bulls, but I don't think that it puts him ahead of AAC (Adam Ashley-Cooper) or Horne, yet. In any case, I wouldn't be restricting the 13 jersey to those who have been playing it regularly (as long as the contenders are capable of playing it well).
For the Wallabies, I think the Genia, Cooper, Hynes, Ioane, Mitchell, O'Connor all look good for a starting spot based on form - with one other player coming in depending on where Deans deploys those 6. There is some youth and inexperience there but what have we got to lose? The Wallabies didn't win squat last season so why not mix it up and let the form players have a go?
How about 9. Genia 10. Cooper 11. Mitchell 12. Fainga/Barnes/Cross 13. Ioane 14. Hynes 15. O'Connor as a starting option?