get a fitting done..... i have the same inseam and ride a Argon18 E-112 in a size Medium, seems small yes, but very comfortable, more likely because of my torso or upper body length.. granted all bike makes will have different measurements, so all the more reason for a fitting. it took about an hour and a half to achieve a nice flat back aero posistion and to get comfy at the same time... well worth the time and money.
the fitting was a critical part of my purchase, its kind of like buying a car without sitting in it ... especially when your spending big coin on a bike...