15", most definitely for what you plan to do. Emphatically recommend you choose customized and get the Widescreen/Antiglare option, your eyes will thank you for it. Just an extra $100. If you can afford it, get the 2.6GHz CPU, as that gets you twice the unupgradeable VRAM for the high power GPU: better graphics performance, especially with external displays. Get stock RAM and HDD, the regulars let you upgrade that at leisure and on your own disbursement schedule later on.
The 13" are castrated enuchs that have a single piddly integrated GPU and shared RAM, with pathetic graphics performance. Great for chatting in Facebook, though. This applies to the regulars, the Airs AND the Retinas, btw.
The MBA really is not cheaper than the MBP. To upgrade the Air to similar specifications comparable to the 15" MBP you will have to spend $500 to get to 512 GB storage, $100 for a faster CPU which still will be slower than the MBP, and $79 for a CD drive. Total: $2278.
Admittedly the storage will be SS but video can have very large files and that will be a high priority requirement unless you want to be transferring data to an external HDD all the time. The comparable MBP will be $479 less. Add the antiglare high resolution display for $100, your still ahead by $379.
What you have to decide between is SS storage vs a superior display. As a part time video editor, I will always choose the display over speedy storage.