Titanfall has the edge, but I think this will be close. I'm not certain if Titanfall will break 3 million lifetime because the best selling next gen game (COD Ghosts PS4) barely broke thru 2 million. Xbox1 total number of consoles will only reach 12 - 15 million by the end of 2015.
Last of Us PS4 may get to 2 million. This is a re-release, but there are lots more PS4 consoles sold, and will probably have almost double the install console base compared to xbox1 by the end of 2015.