I'm not totally sure, but the FN minimi, in any form (249 or 240) will be lighter in the long run because of the lighter ammo. I believe that it is more reliable too, which is why it is, slowly, replacing the 60 as the LMG
in the US Military, for most purposes, but 7.62 is still better than 5.56 for many things, especially when you don't have to carry ammo