Tamron TC's are mad by Kenko, but might not have the same electronics, but glass is the same.
So you are down to Sigma, Kenko, or Canon.
Do you have or need weather sealing? Canon is the only one. Otherwise, I'd go with Kenko. I did buy and return a Kenko 1.4X TC, it locks up a 5D MK III with certain lenses, most noticably the 100mm L. It only seems to do this with the MK III, I have no info about a 6D.
There is always the possibility that a TC will fail to be compatible with a future camera model, so factor that in as well. There are a ton of older third party TC's that are not (fully compatible with newer cameras, meaning that they do not report information correctly, or do not work with certain lenses)