Please - I do need some wise advise 2choose: EF500II/EF600II/EF200-400?

Yesterday I visited a mini-photo-exhibition with Canon pro photo desk - where they showed the one of the two available in Europe a pro-production
Canon EF200-400mm together with the two new
EF500mmII and EF600mmII!
After holding and lifting and handling these three super-tele-lenses and comparing the balance and weight - I felt somehow a bit
undecided which ONE to pick for my - future wildlife and predominately bird-photography that I have never really and truly invested in - to really seriously try to photographing!
The main thing is of course the hand held ability in the field.Comparing the EF500II and EF600II made my somehow a bit surprised by the very little real difference of the weigh-factor comparing between these two long-lenses!
I spontaneously came to the impression and part-conclusion that the new EF600mm f/4L IS USM II is so close in weight to the EF500II that I think that it actually maybe would be even the better choice of the two super-tele-lenses namely
for full frame DSLR?
My present two DSLR´s are Canon EOS 7D/ 5D MarkII.
I am also in need of a new DSLR body - thinking about 5D Mark III OR 1DX OR 1DMKIV - but leaning now mostly to the 5DMarkIII or who knows - maybe the new 6D with the brand new sensor could be even a bit better in lowlight compared to the MarkIII? That was the Canon Pro Photo informer´s personal un-tested impression about the new coming 6D consumer model that is cheaper than the 5DMKII?
Anyhow - my main question is this:
Investing about 100 000SEK on a new super-tele-lens - should you in my shoes - choose EF600mm f/4L IS USM II before the EF500mm f/4L IS USM II or wait for the EF200-400mm f/4L with Extender 1.4x (560mm f/5.6) with the weight around 4000gr according to the un-checked weight said by the Canon Pro Photo informator at the Canon desk!
I must say that holding the new EF600mm II was little like being very positively surprised by the big barrel that is so "light" (there is always a curve of learning and adopting to handle a big lens).
My present "biggest" tele is the old first version of Canon EF300mm f/2.8L IS USM I with both Extenders version III and also EF300mm f/4L IS and EF70-200mm f/2.8L II/ 4L IS also.
Should I sell one of the 300mm:s or one of the tele-zooms according to your valued opinons?
I would very much appreciate your experiences in handling long-tele-lenses out there in the real field work in various weather conditions etc!
Pros and cons?
Pros - EF600mm II
more reach with full frame DSLR and more versatile together with the new EF Extender 1.4X and 2X converters (compared to the EF800mm f/5.6L) that this lens EF600 II, is specially adopted to perform well together with.
Cons - more expensive (but not over-all much more in relation to what EF500mm already costs) and the
weight 8.64 lb (3.92 kg) compared to EF500mm II = 7.03 lb (3.19 kg) the difference is =
0.73kg or 730gr or 1.61 lb
Maybe the longer Minimum Focus Distance (EF600mm)=5.0m vs EF500mm=4.0 m
The EF600mm will be a bit bigger back-pack and to handle and to transporting in airplanes etc
(In addition - to a long-super-tele-lens there might always be the additional requirements of more high-class equipement that will add costs like - I do not yet have chosen a Gimbal or a good and light ball-head (Acratech(Arca-Swiss P0) to my tripod still without a ball-head it is a - Gitzo GT3541XLS Systematic 6X Carbon Fiber Tripod Legs - and I recently found this only carbon built Gimbal head and therefore thinking about the carbon gimbal head made by Sirui GP-20 with having low-weight pros compared to Wimberley II about 0.4kg!
I am also considering the need of purchase the Gitzo Monopod called - Gitzo GM5561T Traveler 6x Carbon Fiber Monopod - Supports 55.1 lbs (25kg) but it is really expensive!(I have already the RRS Monopod head - with quick-arcs-swiss-quick-load).
So if there is someone with some time to answer my questions and maybe who are familiar with these choices - please do share your experiences with me!
Wishing you all the very best and may you have great shoots out there in the field with photos that makes difference for you!
/Charl