I have an 11" and my dad has the 13", so I have experience using both.

I find the 11 inch is such a nice size for portability, however it's extremely annoying to work on because the screen is so short. The lack in vertical space is truly annoying.

The 13 has a lot more pixels which is nice, however in my case, the higher pixel density is harder to work on because my eyesight isn't as great as it should be and the denser screen makes focussing more difficult for me. Most people probably wouldn't mind though and may even prefer it.

I'm betting that the 13 is a better choice for you if you have any desire to be productive.