Is the 100-400 mark 2 still coming out this year?
Seems like every few months there's a rumor that it's around the quarter, but 2013 is almost done and still nothing from Canon.
The last I saw was that it was due out "around the time" of the 200-400 release. Maybe I missed something...
Depending on which crop you are using (ff or 1.62x) really depends on which lens would be of most benefit to you. The current 100-400L is still a great lens, although it could really do with an overhaul. It's AF is quite slow and it's IS system is one of the original systems implemented. Things have moved on, but it's still a great lens. Early copies are very good, then there were a lot of QC issues. The later ones are close in sharpness to a prime and have a lot more versatility.
If you are on a 1.6x crop, then seriously consider the 70-300IS L. It's newer, sharper, contrastier and better colours. It has faster AF and far better IS system. It's lighter and smaller too and on a 1.6x crop it's an equivillent of a 115-485mm, which is longer than the 100-400 offers on a full frame. The 100-400 is a little shorter than 400mm at the long end, it's closer to 380mm.
If you buy second hand, actual hands on is a must with the 100-400 as there are a lot of crap copies, then when a mkII is finally released (pigs will fly and unicorns will run), chopping your lens in S/H won't cost you all that much as most of the drop will have already occurred and your cost of ownership will be nearly zero.