"Which of these incrementally better cards is best?"
Look up the AMD and Nvidia naming schemes, that will tell you all you need to know.
chucknificent :
The "x" versions of the R9 cards has the same GPU architecture as the regular version. The x simply means that it is factory overclocked and in some cases more ram is added. For example, I have two R9 270x cards. These cards normally come with 2gb RAM, but I purchased ones that have 4gb ram each. Buying the x version of a card guarantees that you can overclock it to that speed. However, I have seen many people overclock their regular cards to the x card's speed without a problem. They are the same card, just one has been factory overclocked.
Nope.
The X versions do contain more hardware as well as (typically) being clocked higher. Usually its more stream processors and/or GPU cores. The memory doubling isn't correct either, that's a 3rd party modification to the reference design. As for why it only happens on "X" cards, its because why would you do it on the inferior card?