EM is an additive that sweetens but as also mentioned gives a candyfloss flavour if you use enough or too much of it. It can make fruit flavours ping out but be warned it can also mute other flavours.
Using flavourings to sweeten has the benefit/drawback of also adding that flavour.
Sweeteners are usually either sucralose (ready mixed in PG, made by TFA as Sweetener) or Stevia ... where you can buy the crystals and dissolve them yourself in PG.
Which sweetener is the best depends on which taste you prefer.