My Cervelo is a 51. I'm a hair under 5'5" with a 30 inch inseam. I had to get a shorter stem. One of my road bikes is technically a cross bike, so I'm use to twitchy.
As far as SRAM or Shimano... you are using Shimano now. Are you comfortable with the method of shifting that Shimano uses? I'm not talking about the components, I'm talking about the way you actually use your hands to do it. If that method works for you, why switch? Upgrading within the method, sure, but why learn something new?
I got my bike with SRAM because that was part of the REALLY good deal. But my other two road bikes have bar end shifters, so I was going to have to adjust to something new, no matter what.
Veronica