Okay, here goes...just buy a Retina MacBook Pro. Let me state my credentials. I am not a pro photographer, though I feel like I have some
. I AM
a professional software and systems engineer who owns and works from Macs except when I am forced to use Windows when working at a client site (DoD). I just upgraded from a 2011 15" MacBook Pro (which I had swapped an SSD into) to the 2014 latest/greatest 15" Macbook Pro (MGXC2LL/A). I also have owned a 27" Core i7 iMac and have built a core i7 Hackintosh. 8gb of RAM is plenty for photo editing. That being said if you find a good deal on one with 16gb don't say no. An SSD is going to give you your single biggest performance boost. A quad core processor (like in the 15" MacBook Pro line) is going to give you more performance when importing/batch processing/exporting than the dual-core in the 13" models. For editing a single photo either is fine. Don't overlook buying a refurbished MacBook Pro http://store.apple.com/us/browse/home/specialdeals/mac/macbook_pro/15
. In fact, this one looks particularly sweet http://store.apple.com/us/product/FGXA2LL/A/Refurbished-154-inch-MacBook-Pro-22GHz-Quad-core-Intel-i7-with-Retina-Display
. The only issue with a 256gb drive / external drive is that you will need to do a good job of manually moving older photos to the external drive. Thankfully, Lightroom 5 and SmartPreviews makes working with images on an external drive really easy. Depending on how much you shoot, you should be able to keep the last 3-6 months on the internal drive.
MacBooks are tough, fast, and OS X is reliable. You'll need to upgrade machines less often. You really won't regret spending the extra money.