If you're just starting out, then try using the rentals for a while first. You may not necessarily learn about what you like, but you will start to notice things you don't like in a frame/fit. This can help you narrow down your choices.
To start, fit is more important than stiffness, when you are starting out. Most people tend to upgrade their bike/frame after 2-3 years. The most important thing to consider for your first bike is getting a bike that is CLOSE to your ideal fit. This leaves you room for stem/bar/seat tweaks to find out what you really like. I like to think of it as you should get a frame that allows you to stretch a little as you grow into the sport. In other words, a frame that accomodates a comfortable position now, but will allow you to go longer and lower in a year or so.