Properly made, there's very little difference in the two. If the strings weigh the same (mass weight), the speed will be pretty much identical. Flemish seems to be a little quieter in my experience. Endless seems to be more consistent from one string maker to the next. Flemish is more adjustable.
With the same number of strands, same size/length of center serving, etc. endless and flemish will weigh about the same. I just wieghed a couple of strings I have lying around--they aren't exactly the same, but close. The flemish string weighed less.
Maybe O.L. will chime in--I believe he's done a good bit of comparison here. I'm sure there's a reason he went with flemish, as do at least some of the top IBO shooters.
Either way, IMO there's not enough difference that 99% of us can tell a difference in the way they perform (properly made). I make both, and shoot flemish.
Chad