I've got both the Sigma EX and the Kenko Pz-AF 2x … optically they're much of a muchness, but as noted above, the Sigma is limited to which lenses it'll match with. As with all 2x teleconverters, the effective loss of two stops of light will restrict which lenses will reliably autofocus. My 70-200mm f/2.8 is no problem but my 150-500mm f5-6.3 can struggle with only a 1.5x adaptor fitted, let alone a 2x
Also, if it's of any consideration, not all converters will power a lens with a built-in focus motor and not all lenses have an a/f screw-drive