Depending on what camera you're using you will lose auto focus because some bodies only AF up to f5.6 and some only AF up to f8, attaching the 1.4x to the 100-400 tells the body its now a ~f8 lens and attaching the 2x tells the body its a f11 lens. You can get around this by putting a bit of tape over the adapter that transmits that information or by using a non-canon adapter.

To address the other question '2x vs 1.4x', the 2x very noticeably degrades image quality (among other issues) while the 1.4x doesn't impact image quality as bad. Some would say the 2x degrades quality worse than if you just increased the resolution of the photo 2x in post-processing where the 1.4x actually fares better than if you did 1.4x magnification in post.

Depending on what camera you're using you will lose auto focus because some bodies only AF up to f5.6 and some only AF up to f8, attaching the 1.4x to the 100-400 tells the body its now a ~f8 lens and attaching the 2x tells the body its a f11 lens. You can get around this by putting a bit of tape over the adapter that transmits that information or by using a non-canon adapter.

To address the other question '2x vs 1.4x', the 2x very noticeably degrades image quality (among other issues) while the 1.4x doesn't impact image quality as bad. Some would say the 2x degrades quality worse than if you just increased the resolution of the photo 2x in post-processing where the 1.4x actually fares better than if you did 1.4x magnification in post.