Actually due to the holiday digital bundle that Reach received in 2011 it's probably about tied for sales with GT5. If I remember my calculations right it roughly sold 1.1 million in digital copies during the time that it was bundled and that doesn't include the other 2 or so years it's been on demand (which is increasingly becoming a bigger market on the 360). So best bet is to add a couple hundred thousand onto that. Overall I would estimate Reach at around 10.6 million (taking the very low end of sales)
Either way though one game didn't meet my sales expectation (Reach) whilst the other one (GT5) genuinely surprised me. Congrats to GT5 for being the first PS3 exclusive to cross 10 million.