It's probably pretty difficulty to pick up the finer differences between bicycles if you haven't ridden quite a bit. I know that when I first started shopping, I could tell basic ride characteristics but many of the bikes felt the same to me. The only real difference I could see was the price and brand. As I rode more ( I have about 6000 miles on 2004 Trek 2100), I got to use to the handling and ride characteristics of my bike and using my bike as a reference point, it's become easier to tell the differences between bicycles. It may be hard for someone with no reference point to spot some of the finer differences, but you should be able to feel how they handle and how smooth they ride over rough spots.

I would first decide if you want a relaxed (pilot, roubaix) or aggressive ride (trek 1000 through madone) and search for a bike that meets that need or can be setup to meet that need. Ride them all and write down your impressions on how they rode, what the price was, what size you tried, and how you felt about the fit, such as being too stretched out, too compacted, seat to bar drop was too high, etc.

Once you know how basically how they ride and what they cost, work with the LBS to resolve any fit issues you had. Also, ask how the LBS would handle a fit issue. When I bought a new bike, I wanted to try a shorter stem and they told me I had to buy a replacement stem at full retail. I mentioned wanting to switch to a shorter stem at a different bicycle shop in town and they offer to trade my stem for a shorter one, no problem. Some places are more flexible than others... If the fit issue cannot be resolved so that you are 100% satisfied, go to the second bike (or second LBS) on the list.

When I bought my bike, it was based largely on how it fit, rode and looked. Like you, I thought that there were several bikes that would work and in reality, there are. The question is which do you like the best and will the LBS you are purchasing the bike from help you on fit issues?