Basically all TC will work as long as the manufacturer specifies that it is compatible. But anything goes wrong with the L lens and the cause of it is the TC. Canon will not honour the warranty. That happened to me. I used the tamron on and it didn't fit that well so I used a bit of strength and there was a crack on my L lens. Brought it back to canon. Must pay to get it fixed. Just thought that you might want to know.
Most would shun the 1.4x Extender due to costs and the fact that it will work ONLY AND ONLY WITH certain models of L lenses. Others might end up damaging the TC as the front element protrudes out quite a far bit.
Optically wise, both are pretty much the same except for the weather sealing in the MKII model.