2012 or newer, or whenever they changed to the new thinner design. Fusion drive or SSD makes a big difference to speed, fusion drive is cheaper. Processor speed does not seem to make a big difference – if you really need it you will already know because of the stuff you are doing. Base 8GB memory is fine, third party upgrade is easy on the 27″ but not on the smaller one – this alone makes 27″ the better buy. Retina display is nice to have, but you won’t miss it if you are looking to save money.