Used Giant Rapid or new Trek Lexa SL?

I had a very unpleasant experience with TREK, The front hub cracked while riding in perfect conditions, and Trek didn't really care. Go with the Giant or another reliable brand such as Specialized or Cannondale.

Keep in mind that although used may look enticing, unless you have all the tools for a service and know what you're doing, you probably will start off with an overhaul, so new cables/housing, clean and lube all bearing surfaces/pivot points, new chain. If the maintenance has been less than stellar (and unfortunately this is true of the less expensive bikes since more than likely they have been owned by someone that only ride occasionally or another beginner than doesn't have the knowledge/tools), which means that you may need a new cassette and chainrings. All said and done, you can be another $2-400 into the bike quickly. Most brand new bikes comes with some kind of tune-up included, either first tune up, or first year, or something like that. If not, negotiate that into the price. The margin on bikes are fairly low, so not much the LBS can do about the price on the bike itself, but margin on accessories (shoes, helmets, etc.) is much better, so something you can get discounts on those thing. Labor is a cost that is fixed (you pay the mechanic by the hour whether he/she's working on something or not), so getting free service included is usually a win-win situation for the LBS and the buyer.

If those were the only two bikes to pick from I would get the trek for no other reason that it is a road bike. The Giant is a hybrid and if you are interested in doing tri's I don't think you would be happy with it.