I still don't think GT5 will break 10m, but that also depends when Polyphony makes GT6, which could take at least half a decade...
If Halo Reach is at least on par quality wise with Halo 3 (preferably even better), I'd be disappointed, if it doesn't manage to sell more than Halo 3 LTD.
So nah I think Halo will win.