100-400 or 70-200 with x2 extender

You should buy the 70-200 F4L non IS. This lens is amazingly sharp, relatively fast and very light. It also is a bargain and would allow you to keep the 100-400 also. With the ever improving high ISO performance of these new bodies this lens will become very popular I would think.

Thanks everyone, it looks as though I will be hanging onto the 100-400 which has served me well and look at a 70-200 when I have some more funds

the 2x is a fun little toy to bring along just in case you would rather lose the IQ at the time of exposure than later on during cropping and post processing. I use mine to double the 100-400 to look for wildlife but rarely for actually capturing images. get the 70-200 2.8 for the bigger aperture, keep the 100-400 cause it is a great lens, but don't expect much from a zoom and a 2x.