Just curious for 15" rMBP owners, how portable is this thing? I'm looking to upgrade from a 2009 13" MBP, and I'm really big on portability. The reason I'm considering getting the 15" is more about the processing power & graphics than about the actual size. I know the 13" is obviously more portable, but is there a huge difference in portability between the two? The price difference would be $230 between getting a new 2014 13" rMBP and a refurbished 2014 15" rMBP.