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!