I agree with Adam. Pay for the 1TB upgrade, especially if you are going to shoot RAW video on the 5D3.
I also think you need to consider the Retina resolution on the 13" MBP makes text VERY small. If you don't have good eyes/corrective eyewear (presbyopia etc), software menus can be tough to read. The 15" has that extra bit of real estate. I definitely agree with your idea of going to an Apple Store to view the screens with the intended software.