Going to look at a display model - what should I compare?

In my opinion you should also check the responsiveness of the application e.g.
Loading, closing and general usage. compare this to the non retina Macbook Pro and see which one you prefer (which is faster) Hope this helps.
Compare non retina apps on a macbook pro (retina) to a macbook pro (non retina) to see which has better performance

Also applications like google chrome should be optimised for retina over time.

 

I was at an apple store last night and had a Non-Ret directly behind a Retina model and I must say, I couldn't tell the difference. I even happened to find the same photo in iPhoto and brought it up full screen on both and couldn't tell the difference at all. I guess I'm not sure what all the hype is for the Retina display, maybe there is a difference, but just normal usage I couldn't see it.