I went through a similar thought process when the 70-200 MKII first came out. I'd recently purchased a 300 (MkI, the MkII wasn't out) and was using that with a 1.4x extender, so the 100-400 was getting limited use. It didn't quite go to plan though, as the funds form the 100-400 went on a 24 f/1.4 MkII instead, then the next set of funds went on the 5D MkIII
. Eventually, I did get the 70-200 MkII last January, just before going to Finland and I recently got the 2x MkIII extender. I haven't tested fully, but so far I am happy with the results. My reasoning was that the 70-200 could double up for use in low light (albeit shorter), so was more flexible for my use. I think the weight difference between the two is minimal (especially when used to carrying the 300). When I need to travel a bit lighter, I have the 70-200, if I need higher IQ or more responsive AF, then I have the 300, although, I would like something longer, so that I don't have to use an extender as much, but my funds have been diverted to something else instead.