Hi Cees.. good question. I also run both. I think they're both about 
equally capable, but differ in their manageability.

ezmlm is great for lists you set up and then forget about; they just 
work, no intervention required. It doesn't come with any decent web 
interface that I know of, though there may be one out there. (I've seen 
the one at discuss.squeakfoundation.org.)

Mailman of course has a rather good and complete web interface for both 
admins and subscribers. Many knobs and options to twiddle to get things 
just right, but it's easier to come back to it a month or a year later 
and figure out how to do something.

 From a user point of view, unless there's a better ezmlm web interface 
available I guess I'd vote for mailman everywhere.