Not all brands are standardized when it comes to sizing. A large in one brand may be the equivalent of a medium in another brand. I am 5' 9" with a long torso and short legs and the cannondale q5 medium was a good fit and I believe that was 18.5 inches,however I also shopped a Bianchi camaleonte and its frame size was 17.5 with the next up being a 19. The 17.5 was ok but I noticed I was too close to the handlebars and may require a stem extension or adjustable handlebars. The cannondale was a good fit due in part to the angle of the top tube and the angle of the stem and handlebars but I favor the bianchi for now. To answer your question, the fit is most important and if it is good you will want to ride more. Make sure you have at least an inch clearance and proper distance from the seat to the handlebars and this is something only you can determine by test riding each bike. Some bike stores have equipment to determine the proper fit and can charge for this but if you buy the bike they will deduct that from the price other stores may not. It may be worth it for you to consider getting professionally fitted. Good luck and stay safe on the road