I think for the classroom library that students have access to, the non-leveling choices are the most appropriate. Now, you might keep those leveled books in a place for you to access them for your groups. The thinking is that we want kids to develop the sense of "good-fit" books by choosing them from non-leveled books. There are great lessons for helping kids develop that skill, so that when they go to the library (school or public), they'll be able to choose books there, as well.
Joan tells the story of helping a student in her class and his mom get cards for the public library and arranged for them to be able to catch the bus to go there. When she saw him after the library visit, she asked him what books he had checked out, he said, "None, they didn't have any blue tubs." She says that really helped her become convinced that sorting by topics was the better way.