Figuring out what was right for ME took a good month of reading and research to understand some of the differences between bike designs, including such things as frame, suspension, gearing, overall weight and foldability (and I still have LOTS to learn). My own budget and riding intensions narrowed the list, along with local availability. Test rides prompted a decision, although I still wasn't 100% sure at the time.
A good local bike shop will work with you to identify the best "stock" bicycle, and possible upgrades to better fit your needs. With more emphasis on customization, Bike Friday will do the same -- identify a basic model, then build it for you with the components that best suit your riding.
Be prepared to prioritize your needs, and to make some adjustments after riding for a while.