ABSPATH or __FILE__?

For my own projects I would choose dirname(__FILE__), also there is a new constant in PHP:

__DIR__ === dirname(__FILE__)

I would personally prefer dirname() as it is always guaranteed to give me the correct result, while the ABSPATH method relies on a fixed theme path and theme name that can both change.

By the way, you can use __DIR__ instead of dirname(__FILE__).

• The path to the "wp-content" directory and its subdirectories can be different in a particular WordPress installation. Also, using the WordPress internal constants (such as ABSPATH) is not recommended. See the Determining Plugin and Content Directories WordPress Codex article.

• Since PHP 4.0.2, symlinks are being resolved for the __FILE__ and __DIR__ magic constants, so take that into account.

  Bottom line: To determine the absolute path to a theme directory, I would suggest to use the get_template_directory() function which also applies filters and internally combines get_theme_root() and get_template().