Pillow is a PIL fork created to add new features. setuptools support was also added. A more frequent release cycle was also promised. With Pillow you can have PIL as a package dependency in setuptools and virtualenv. That means less clutter and robustness for the development. :-)
Pillow allows you to continue to use import PIL , so there is no need to change your current PIL related code. 0 migration overhead.
Archlinux already dropped support for PIL in favor of Pillow.