MEM or DMEM - why sometime MEM and sometime DMEM? (Aug/13/2005 )

Hi
I want to know why we sometimes use MEM and sometimes DMEM in cell culture? Which characteristics of cells determine the medium that we should use? I use MEM for HEK2 cells and DMEM for Cos7 cells, However I don't know the reason!

Apparently DMEM is super-MEM...meaning all the nutrients etc are more in the DMEM formula....also according to ATCC DMEM is for a 10% Co2 incubator and MEM for 5%...I use both in a 5% and it works fine..