[antonio7397]

Ching, could pls advise which one is better?

[wesleyyau]

It depends on your usage.
Most of the people choose 1.4x, because the image quality drop is less than 2.x
And for the camera that's not 1-series, the AF only can operate on the lens that aperture is larger or equal to f5.6. So, that means if you use 1.4x on f4 lens, it's still has AF, but if you use 2.x on f4, no AF at all (assuming you are not using 1-series camera).
The price for 1.4x III and 2.x III has the same according the MSRP.

[antonio7397]

thanks ching, i have all lens @f2.8. that means, I can use 1.4X with AF function Right?

[Obi-Wan]

both 1.4X & 2.0X extender AF can work with f2.8 lenses, but beware some lenses cannot be used because the rear glass element can block the usable of extender, for details please check
http://en.wikipedia.org/wiki/Canon_Extender_EF