At the time of this post vgchartz lists PS3's market share at 21.62 to the 360's at 29.78 million. A difference of 8.16 in favor of the 360 BUT what this doesn't take into account is dead consoles. I'm sure that tons of 360's have died and the owners simply bought new ones.
Basically we don't have a current realistic analysis of their actual market share but going by the recent numbers of video game sales I would suggest the PS3 and 360's market share is actually close to being equal and I think the developers know this.