ME3MP is not just a shooter. You have power oriented and melee oriented characters as well as the possibility of running different power and/or melee builds on just about any characters. The possibilities are limitless.
But even for those who like to go gun heavy, most guns are so unique in their own right; take the Reegar and the Venom for instance, they function completely different and feel completely different, even though both are shotguns.
DAMP is more loot oriented than ME3 and all weapons feel just the same, they just have different skins and stats and changing weapons has no impact on playstyle at all, with the exception of the Griffon bow.