Help selecting a Kenko TC for a EF 70-300mm IS USM/t3i

I cant decide which to go for bird photography. Anyone use any of these? does the af work in good to perfect lighting?
Also is it true that 1.4x produces better quality exposures over the 2x?

Thanks to a person at B and H-
1.4x loses 1 f-stop of light (98-420mm f/5.6-8);
2x loses 2-f-stops of light (140-600mm f/8-11);

I've tried them with my EF 70-300mm f/4-5.6 IS USM and in general the money would be better put towards a longer lens.

The IS seems to keep working ok with the TC. The viewfinder gets fairly dark. Try it using the depth of field preview button to see how dark f/11 is. AF will try to work since the TC isn't telling the lens to turn of AF since none of the Canon 70-300's have the extra contacts for the TC to tell the lens of its presense. The AF won't work properly though. It oscillates and overshoots and generally fails.

The 2x will magnify the faults more than the 1.4x so the image quality does get worse. In both cases, the TC is expanding the image circle and therefore magnifying the problems with the main lens. The pictures get a bit soft. If you focus carefully with live view and use a sturdy tripod, you generally will get better pictures than just cropping but you'll have to decide for yourself if the remaining quality is adequate.

I decided on the 1.4 for better build quality and better hopes of the af working.
I tested it this morning, and im kind of happy with it. The Af works in descent to good light.

I wish i could save for nicer lenses, but im stuck with what i have untill at least tax time, and thats if my wife finds it in her heart to let me buy L glass with lower Af's (which im sure she wont).

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