there is no real difference between .esm and .esp except for the following:
a) .esm will load before any .esp
b) .esm will let other modders make mods based on yours
c) it will be easier for you to patch your mod without touching the original .esm and get feedback from players and change things accordingly
d) You can have multiple changes in one mod by making many different .esp’s like many of the mods on nexus have, take for example Climates of Skyrim that have many .esp’s that only changes smaller things like darker dungeons and darker nights.