Canon-wise, and MWO-wise, they took up the same amount of space (so 14 slots each). However, Endo offers more of a weight savings than Ferro, so if you have to pick one or the other, you would NEVER pick Ferro.
How about reduce the slots for Ferro from 14 to 12? Now you get two more slots if you take Ferro over Endo, which could pay off depending on what your build is.
Now the choice isn't automatic and both options offer something different.
Edited by Dawnstealer, 15 September 2015 - 12:20 PM.