PS3 for me. Would have been the case no matter what to be honest. Like still to this day Xbox prompts in GTA gameplay freaks weird lol. Grew up with 100s and 100s of hours of GTA on PS2, it's where it 'feels right'.
And I agree with above, the gameplay being PS3 is an encouraging sign. Though as I said even if it wasn't I would have got it for PS3
Although R* said they are identical in a Q&A so maybe next video will be 360 to prove that.