New Giant Trinity Advanced or used Cervelo P2?

While there have certainly been advances in bike design/construction over the past 4 years, you are looking at 2 fantastic bikes. The question is always, which is best for you! If you are able to get a "diagnostic" fit from a shop, that should greatly help you decide if one frame is a better fit for you than another. I would always prefer a bike that fits me/handles well, over one that might be a tad fancier, nicer, newer etc.

Just a few pointers when purchasing a used bike (we deal with tons of them here at MBS): inspect the frame closely, especially a carbon frame, for any dents, cracks etc. In many cases, the owner might also not know they are there! Pay close attention to the state of all the bearings on the bike, the true of the wheels, etc, since these can lead to repairs that would reduce the cost difference between new and used. If you aren't confident in your own abilities, a shop I used to work at would always offer for folks buying a used bike to meet the seller at our shop, and a mechanic would check it over free of charge, your local shop might be willing to do the same. Also, take into account the cost of adjusting the saddle/stems/handlebars to get your preferred position, as many shops would at least offer you a discount on this gear with the purchase of a new bike. But if you do your due diligence, you can find fantastic deals on lightly used equipment! Best of luck!