It's hard to make that prediction, we don't even know if Titanfall will succeed. It could end up being mediocre, or the servers could have massive problems at launch, there are many variables. IF Titanfall succeeds, it can reach the 7-8, maybe even 10 million mark. I expect (hope) inFamous sells about 5 million.
But, I think Second Son will push more hardware. In some ways, it's the first "true" next-gen game on the PS4. And it's exclusive and shit, so there you go.