I'm 5'10" and a 56 is almost like a toy bike to me, so yeah, it's all about leg and torso length. If you can afford a fit from a recommended fitter in your area, I'd do it (so much less than the loss if your new bike isn't right!). Regardless, for sure go on a couple of long test rides before committing, and if there's another shop or two to try, I highly recommend thinking outside the Trek/Specialized box. Especially if you don't need/want a WSD bike, you may find a shop who will take more time and care with you. Or not...but have fun!
Sit bones = ~135 mm, saddles that work ~ 155cm/6.1 in wide
2003 da Vinci (custom road/all-rounder)/Terry Butterfly Ti
1994 Gary Fisher Nirvana (vintage MTB/commuter)/Terry Butterfly Chromoly
1991 Terry Symmetry (NOS frame/fork, project in progress)
1973 Raleigh Super Course (project in progress)