Thanks for all the replies everybody, but i was wondering what the difference between the two were. Does anybody have any hands on experience with the two to compare?
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Yes, I have both of them. I usually go with the BH-55 on my Gitzo 1257 and it's a good match. I like the tension knob for speed of operation -- it's handier than the lever on the BH-40 for me. But when I need something smaller and lighter I use the BH-40.
I also use the BH-40 on my Gitzo 1027 when I need something light but functional. If I need something SUPER portable (like a week long hike in the mountains) I'll go with the 1027 and a BH-25 (which is *just* adequate for the 1D mark II and a 70-200 f2.8L IS... but only if it's not windy).
Anyway if you need portability, the BH-40 will be better. I took it on the trip to Antarctica and it worked fine. But my biggest lens is my 70-200 f2.8L IS. Your 100-400 isn't much heavier. If you went with a 300 2.8L or larger you'd probably want to go with the BH-55.
As to the weight difference... the difference is a full pound. When coupled with a ~3 lb tripod, the difference between 4 and 5 pounds total weight is definitely noticeable.