Halo is a faster seller than GT
And nowadays a better seller aswell.
Its not that unfair a comparison, but like i saw GT5P came out before GT5 unlike ODST which came after Halo 3, had the ODST come before Halo 3 kinda like a Halo 3 Prologue it would have sold better than GT5P.
GTAIVs expansions have sold 2.5 million on disc aswell, thats not counting how much its sold off of Xbox Live, PSN either. And people were overall not all that impressed GTA4. And again that came after.
So while GT5Ps sales are impressive its nothing out of this world, if MW3 Prologue came out in January its top that 5 million in a week.