Earlier today, Microsoft claimed that 40% of new 360 owners got GTA IV. We're still waiting on official NPD numbers, but Gamestop has released their sales figures, which should be a good estimate of the real sales.
Gamestop is reporting that 64% of Grand Theft Auto IV sales were the 360 version, while the remaining 36% were the PS3 version.
That's a performance of 2-to-1 for the 360 version. Level Up, who received the numbers from Gamestop, notes that this might represent an underperformance for the 360 version due to its larger install base.