Just frame I assume? Which one fits you better? The one that lets you get in a better position will give you a better time. All things equal as far as geometry goes, my personal choice would be cervelo if you can swing it. Tested and true frame, I am sure there are plenty of people who like the trek too, can't go wrong.
The two bikes are probably similar in aero-ness. My advice is to get the one that fits you better. Cervelos tend to be a little longer and lower (aka a more aggressive fit) that not everyone can manage (e.g. if you're long legs and short torso). The P2 isn't bad compared to the P3 and P4 though.