What about first getting a proper bike fit, if not at the shop with the Trek, than an LBS that specializes in fitting. Opinion varies, but I see a proper fit being done initially without a specific bike (e.g. on a fit bike, like Serrottas). Once you know the ideal seat height and saddle setback, you can than look at the stem and handlebar height for reach.

While you can continue to have adjustments made between the 56 & 58 at the shop, I'm thinking at some point you'll just say yes to one of them and may regret it later.

Fit is so important...