I have a 15" MacBook Pro, and I find that I have plenty of screen space for development - most of the time.
Typically, I have one source file open for editing, and part of another document visible for reference. With this setup, I could easily work on a 13" model. The problems occur when I want to have two documents side-by-side at full size. Here's a screenshot of how this looks with Xcode running full-screen:
The two documents aren't wide enough, so I have to scroll more frequently. In order to get the documents to a comfortable width, I have to hide one of the sidebars. On a 13" screen, you would probably have to hide both. If you use two full-size documents frequently, this could be a problem.
Another case to consider is editing non-text files, such as XIBs and Core Data models. The editors for both of these add another sidebar, and the Utilities sidebar (on the right) is much more important for these modes. When working with XIBs, I can fit a single window in the editor pretty well with all sidebars and no assistant editor. On a 13" screen, I would probably hide the navigation (left) sidebar.
In conclusion, the 13" MacBook Pro will have enough space to work with, except when you need two documents at full size. You may have to hide a sidebar occasionally, but the navigation sidebar usually isn't necessary.