Not much difference, a slight weight difference and bellows extension. They make an extension for the Chamonix as I have heard of lengths of 450mm. 300mm at infinity is about the limit for my HZX even with the Nikon M which is a compact and light lens.
I made a close-up photo of my blooming snake plant with my 210mm and had some extension left over. To get closer, I would be fiddling with the front standard and using the axis tilts. This does not lock down as tight as base tilt, especially with a heavy lens like my 250 or 210mm.
I am still happy with it.