The best thing you can do is go to a store and compare them. Some find 11-inch notebooks to be too small, and some find 13-inch notebooks to be too big. I would personally go for the 13-incher -- it's the perfect sweet-spot for me -- but the "try before you buy" approach is applicable in this case.
If you plan on using it often in the tablet format, go with the 11". Web browsing, creating documents, light games and other basic stuff, you'll be happy with it. If you do photo editing or any kind of graphic design stuff, more screen real estate might be good.