I don't think they delayed it in response to GTA. ESPECIALLY considering GTA isn't on the next gen systems. I think the game just wasn't coming together the way they needed it to fast enough to hit the original date. This being the first in what they want to be a big new franchise, they have to come out of the gate strong. 5-6 months could conceivably give you enough time to polish it to that point. Especially if it was shippable as it was.
Apparently the mock reviews for the game were fairly low when it was tested in September. I've also heard that they were having running way behind schedule porting the game to PS3/360, but I don't know if there's any truth to that.