Also remember, 11" is 1366x768, 13" is 1440x900.
For me having had a 15" MBP for over 6 years now with a 1440x900 display and also for the last few having a 15" Windows laptop with 1366x768, I could never use the lower resolution for anything other than consuming information like I do on my iPad. I would say get 13" unless you feel the physical size of the 13" is too large or you don't want to spend the extra money. Also battery life on 13" is longer.