Recoil Spring Weights

Can someone please explain to me like a caveman what the differences in the spring weights are on guide rods? I am looking at getting a gen 5 glock 19 or a glock 45 and want to put in a pure tungsten guide rod but can't figure out which weight to get 15lb or 18lb. I know it is related to ammo grain weight but not sure which would be best for me.