Em is arguably the simplest easiest open chord to fret. There are multiple fingerings used regularly to affect the same voicing. Those two are both most common. Neither is wrong.

Fingering it with fingers 1&2 is easiest for rank beginners with no disadvantages changing between other open chords and so is what is most commonly taught initially by common convention.

If you learn to play it with fingers 2&3 and are more comfortable with that, play it that way. Neither is more correct or cleverer. As you progress you'll play it both ways adaptive to facilitate the progression/song you're' playing. P.S. Same applies to D. 1,2,3 fingering is easiest which coincides with fastest rate of progress which most beginners need to perceive to sustain motivation.