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HI, I would like to know what is the difference between these 2 models, they are similar in power but there are differences between both models 2.7 i5-4570R / 2.7 i5-1536 THANKS

EMC 2638 21.5” iMac 2.7 GHz Core i5 (2.7 GHz Core i5 (I5-4570R) only offers the intergraded graphics engine (Intel Iris Pro 5200) EMC 2742 21.5” iMac 2.9 GHz Core i5 (I5-4570S) offers a dedicated GPU chip (AMD GeForce GT 750M) as well as the intergraded graphics engine (Intel HD Graphics 4600)