Most people say that two 1.4x teleconverters gives you the better quality over a 2x. However, you can't stack 2 Canon 1.4x teleconverters because of the way they are built. A very good Kenko pro 1.4x and a Canon 1.4x can stack on top of each other. However, if you have to use more than just one 1.4x teleconverter...you may have choosen the wrong lens. Only f2.8 prime lenses take a 2x ok. The quality is greatly diminished. Whereas a 1.4x barely takes away from a prime lens. I've used the 2x and would only use it in a pinch if I had to. I think I would rather use a 1.4x and crop the picture further than use the 2x. To each his own. (by the way, you'll lose autofocus with a 40D and a 2x unless it's a f2.8 lens, in which case the autofocus will be really slow)