We have unit sales through the same period for some of these games too.
GTA IV (both versions): 525,000
Mario Kart Wii: 453,000
Wii Fit: 282,000
Link
So assuming both GTA versions were roughly priced the same that would put the split at 58.7% / 41.3% in favour of the PS3 version, or;
GTA IV PS3: 308,000
GTA IV 360: 217,000