I had not noticed the thread! But now I'm here, so I'll kick in my advice.
Frankly, I think you will be fine with either, but I have not used the Sony, so I will confine my advice to generalities.
I would recommend something that is not so heavy that you will not want to lug it around. You will have to carry the equipment over rough terrain, up hills and so on. Carrying boatloads of lenses in these conditions is a pain in the ***.
I would also recommend that your camera system is weather sealed and that you bring some UV filters for your lenses. The weather here often goes from sun to rain to sun again in a matter of minutes, and those times are often the best to shoot in. The wind will also be howling, so a camera may have water hitting it from all angles, so a well-sealed camera is an important asset. The wind also makes UV filters a good choice -- sea spray is hard to avoid near the coast, and in the glacial flats there is often fine blowing sand that will do a number on your lens.
For lens choice -- I agree with your workshop leader -- having access to wide angles like a 16-35 or 17-35 is quite useful, as is having something around 180mm. If you want to shoot birds, something like a 180 with a 2x TC will go a long way. I would not bring a lens bigger and heavier than a 70-200. In fact, I would consider looking at older prime lenses in the longer focal lengths. I don't think the zoom is really that important in these sorts of shooting, but I gravitate towards primes anyway...Having something like a 180mm f/2.8 and a 2x or 1.4x TC will be easier to carry and versatile. Or just the older version Nikon 300mm f/4 with the 1.4x -- that will give you all the reach you need. To be honest though, the longest lens I have ever had is a 180mm and a 2x converter and I have not felt the need for more reach than that.
Basically, my way of shooting here is with the D3 or M8 and a medium format film outfit. I found that this gives me a really good blend of digital versatility and weather protection with the resolution and look of medium format film. Like Charles, I would be happy with the M8 and a Mamiya 7, but since you are focusing on nature photography, I would say the D700 or A900 and the Mamiya 7 would suit you well. If you don't shoot medium format, then maybe the resolution of the A900 would be nice to have. I have not shot with it, so I cannot say, but holding it in the hand, I must say the D3 felt a lot more ergonomic to me. But perhaps that is just getting used to it.
Finally, on the topic of ISO -- 800 will probably be plenty. There will not really be much darkness at all in July. The solstice is June 21st, and even today there is no true night -- I was out at 1am last night and there was still light in the sky. Sunset is now 10:18pm, and it does not get "dark" until 11:30 or so. When Terry is going is even closer to the solstice, so there will be even more light. Most of your shooting will be done easily with ISO 100-400.