Help me decide 2013 Mac Book Pro, or 2013 27" iMac. Pros and cons of each.

If you have vision issues you really owe it to yourself to look at a 15-inch MBP retina.  Combining it with a 27-inch thunderbolt monitor would be the ideal solution for visibility, convenience and portability, but it would stretch your budget by a few hundred bucks.

I have the 15-inch retina with 16GB of RAM and the lowest spec processor and SSD (256GB).  I find that most of the work that I do to slow down the machine is related to RAM, and I use an external drive to hold my referenced images and iTunes content.  The Aperture library sits on the SSD, so I can view thumbnails even when not connected to the drive that holds my images.  I am very happy with the performance of the MBPr with this configuration.

Mark