I'd say those are two different beasts, both bikes with good reputations. As a novice rider, I'd say the most important factor comes down to which bike appeals to you. Geometry wise, the Synapse will offer more long distance comfort, with it's curved seat stay design, taller headtube and longer wheelbase. The Trek's frame design is slightly more performance based, though that's unlikely to make a noticeable difference until you pair the bike with a performance engine and then your talking marginal advantage
Both framesets have a limited lifetime warranty and the same groupsets. Ultimately you want the bike that inspires you to ride.