Telephoto Extender Help

I am looking to purchase a telephoto extender for my 100-400mm lens but just don't know quite enough about them as of yet (still learning).

I have seen many different brands being used here in the forum so I am confused a bit on which one the pick. Also, will this mean I will loose my AF on my 50D if I use a TX?

Any other usefully information would be helpfull seeing how this is a bit new to me.

Thank you