Rumors abound of the imminent release of a 13" MacBook Pro with Retina display (see my review of the 15" MacBook Pro with Retina display). We should know next week.
Like many, I confess to being attracted to the more compact form factor of the 13" model, especially if the display is the Retina display (should be 2560 X 1600 pixels in theory). On the other hand, my needs don’t involve airplane travel or similar, and so I’d take a 17" model (N/A) preferentially to a 15" or 13" model, just for the screen real estate (3840 X 2560 would be incredible!).
Today as I was processing some Nikon D800 files into 16-bit TIF, I noted that Photoshop CS6 was using all 4 CPU cores. Now realize that the speed of the 15" model is about as fast as any Mac today, and it is too slow.
In short, if the 13" MacBook Pro with Retina display offers 16GB memory and the same size SSD, etc, if it remains a dual-core system, it will inherently be crippled for my type of usage compared to a 15" model, and immediately disqualified as a machine for my usage, nor would I recommend it to any photographer working with D800 size files.
In short, if the 13" MBP Retina turns out to be a dual core CPU, then one should really only consider it if size and weight are the #1 consideration. Since the 15" model is already svelte, a dual-core 13" model does not sound attractive at all.