I recommend the 6-core for photographers, or the 4 core if planning to upgrade the CPU in the 2013 Mac Pro to 6/8/10/12 cores.
Apple factory sealed refurbished 2013 Mac Pro with AppleCare [Apple Refurbished, Like Brand New. Factory Sealed. 1 Year AppleCare Warranty]
+ upgrade to 64GB memory for only $474!