I noticed that the 13" MBP has different CPU options for the i7 for the retina vs the non retina versions (2.9-3.6 vs 3.0-3.5GHz). Is the non retina version a older i7 version? and what is the performance difference between the two? I don't need a retina mac because my eyes are not that good. so I could save a ton of money if the CPU's are comparable in performance.