I'm a photographer who currently uses a 2011 iMac i5 (quad core of course) and 20gb of RAM. I need a new laptop, and I'm wondering if I should get an air to supplement the iMac or just replace the iMac with the 13". The 15" is too large to be an option. So how does the speed of my iMac i5 (quad + 20gb Ram) compare to a 13" retina (dual core + 16gb RAM) Is the 13" powerful enough to replace my desktop, edit heavy raw images and often hook up to an external display? Thanks!