I went to gamerankings, told it to give me the top 50 360 games. The 50 ended at 82.500%, might go further than that, dunno. Look up for graph.
The 360 has a great starting year because it's the first next-gen console. A crap load of projects that were for last-gen either went to both or just the 360, as well as the normal thirt party stuff (the dedicated devs release the good games, the first year)
On the PS3, you see the the lack of both of these, and has Sony sells Blu-Ray companies decide to dive in, giving it a high 2nd year as the 360 gets shovelware.