I hope you don't actually care about what GT6's Metascore is Ethomaz? GT5 only averaged at 84% and you love it, that's all that matters.
As for my prediction, I think it will score between 86%-89%.
Wouldn't it be funny if the game actually averages over 90% but Metacritic's stupid weighted average system weighs it down to 89%, it happens. You should change your bet with Sales2099 and instead of comparing a straight up Metascore you should work out the actual averages and see what scores more, GT6 or Forza 5. (I think Forza 5 will score better with critics, between 89%-92%).