Everytime you spend money on one thing you lose the ability to spend or invest in something else. Buy what you need and not what you want.
A lot can be done on the 13". It really isn't all that small. On top of that, OSX is set up to maximize production on smaller screens. That is why you have the ability to have numerous desktops accessible with a three finger swipe.
If you are editing video all day everyday, then go for the 15". Otherwise, spending more money to save yourself a few minutes every now and then isn't sensible. Who sits there and stares at their computer while it renders anyhow?
When considering refurbs, you can get the base model 15" for $1,699 (8gb/256), or you can get the top model 13" with a RAM upgrade for the same price (16gb/512).
Also, buying the 15" puts you in the screen roulette gamble.
Good luck.