You can still upgrade the RAM on the Retina iMac and it's somewhat cheaper that way.
Let's see, upgrade a 2008 MBP with ram and a SSD, money into a 6 year old laptop or springing for the state of the art 27" Retina iMac! I don't think you could ever talk me out of the 27" Retina iMac, I will not be deterred!
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