I'm new to road bikes. I have some questions about three bikes that I have selected. I'm looking at:
2012 Defy composite 2 with sram apex $1849
2011 Defy Advanced 3 with 105 5700 $1799
2012 Specialized Roubaix Apex with sram apex $2200

1. What is the difference in the carbon fiber used in the advanced and roubaix and then the composite defy?
2. Sram vs. 105: just a personal preference? What is a good way to test the differences?
3. Do zertz inserts actually do anything?
4. Both of my lbs have offered test rides, but I don't know how one determines it is the right fit. I keep seeing riders say to test ride, but i'm not sure how to compare each one or what I am comparing.
5. I have bulging discs at @L4, L5, S1 which are managed through stretching, meds and steriodal injections when necessary. Is there anything I should test while on my test rides?

TIA!!