I voted "more for lower, less for higher" with the intent of commenting as follows:
As mentioned, this doesn't always work, but it seems to make me happy and seems sensible.
After reading the whole thread and thinking it through a bit more, though, I would change my vote to "equal for all" becaue of the curve (lower levels go up with less XP) and because I do have a problem with the idea that even though the 20th-level did much more than the 1st-levels (theoretically), s/he is penalized (sort of). However, giving the higher level more XP would only make sense if, as mentioned, there was some reason for the level differential. I like the idea that cohorts, etc., would strike out on their own once reaching one or two levels of the PCs. It makes sense.