Kenko Pro AF 300
1.4x eats 1 stop of light
2.0x eats 2 stops of light.
2x usually degrade the quality to the point you don't really want to use one. 1.4x's are better in this respect.
Here is some side by sides
http://www.pbase.com/fstopjojo/lenstests (bottom of the thumbnails)