I have the 200
/Extender1.4/Extender2.0 lenses, 10D/40D/5D/5D2/7D/1D4 bodies, Wimberley WH-200, Arca-Swiss Monoball Z1 and the Gitzo GM5561T, GT5541LS and GT3541LS.
I shoot mostly birds and sports.
If you plan to use it for birds 80-100% of the time go with the 600 II (3920g). Reach is reach and on a full frame body that is something you will always be looking for. Friends with the original 500 (3870g) have since upgraded to the 600 II because of the 50g difference.
If traveling by plane is a possibility you may want to consider the 500 II (3190g). Weight and size does matter if you want to avoid paying for excess baggage and walking great distances will be the norm. Friends with the 400/5.6 or 100-400 have since upgraded to the 500 II or source the 500 I from owners upgrading to the 600 II.
Ballheads are a bad idea when it comes to lenses longer than a 300/2.8 because you will need to always tighten them when you let go of the camera or else an accident can occur.
Go with a gimbal like a Wimberly WH-200 or Mongoose M-3.6. I've advised friends who suffered from inferior gimbals to buy either two. They're ecstatic, beyond words and wished they switched earlier.
Fieldcraft is gained if you are an expert bird watcher or even a half assed hunter.
200-400 is reported to be within the weight of the 600 II and cost as much as the 500 II. No shipping date has been announced. What I like about it is the built-in Extender and the flexibility of the zoom. What I dislike about it is the reported weight that is heavier than a 400/2.8 IS II (3850g), price and the f/4 aperture.
It also helps if you are physically fit or workout. The hobby will have you lugging around the camera, support, bags, misc items.
Bags I've used are the LowePro Lens Trekker 600 AW II that fits the 800+body+extender with hood reversed easily but is not handcarry compliant.
The ThinkTank Airport Accelerate v2.5 can fit the 500-only with hood reversed. A much larger ThinkTank bag called the Airport Accelerator can accommodate the 600-only with hood reversed without a problem. ThinkTank Airport bags comply to most major airline hand-carry dimensions and also accommodate other items.
If you frequent South East Asia like Bangkok, Singapore, Hong Kong or Manila you can get the lens earlier and cheaper. Let me know if you need help in sourcing from any SEA city.