I think the biggest difference and deciding factor will be clear to you from holding both. They feel different, and since everyone's hands are different, so is their preference on the handles.
Personally, I really don't like the ESEE handles on the ESEE 5 and 6. To me, it's boxy and uncomfortable. My BK2 fits my hand PERFECTLY. Regarding steel and all the other stuff, those things are negligible differences by comparison. The steel is similar, the blade shape is similar, etc. The sheath on the ESEE is definitely better. The pommels serve different purposes. The handles are completely different.
Try and find some at a gun show or something, or like someone suggested earlier, buy one, and if you don't like it, sell it and get the other.