RRS Ballhead Choice

I would go with the BH-55.

I first purchased the BH-40 for use with my 5D and 70-200 f4.  For the most part, it worked well, and is certainly capable of supporting your 70-200 f2.8.  The only problem, for me, occurred when I wanted a very precise framing.  In these cases, I noticed a tendency of the head to sag or droop once it was locked down.  Not by a large amount, but enough to drive me nuts if I was trying to get a very specific framing.  After living with it for a couple of years, I bit the bullet and got the BH-55.  This thing is as solid as a rock.  Set it and forget it.  No droop.

I still use the BH-40, especially when I am travelling light.  I have it mounted to a smaller 2-series Gitzo.  Closer to home, I will always use the BH-55 (currently mounted on an RRS TVC-34L tripod).

Bottom line is that they are both very good ballheads.  I just think the BH-55 is better.  In the long run, I think this is the one I would choose for your lens, particularly if you are also using it with something like a 1D, or a 5D/7D with grip.

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mrainwater.com