In general, anyway you can get the notes to sound is fine.  So, either way is fine.

Backing up and looking at the bigger picture, the overall principle here is to use the most natural and efficient motion possible to play the music.  With that in mind, I encourage all students to put practice time into working on different fingerings.  We typically learn one way to play a chord, then stick with it as long as it works.  But you might come across a chord change in a new song that would benefit from using a different fingering on that chord.  So when you get to that problem, you work on the different fingering & add that to your skill set.

And the sooner you put time into any part of learning, the sooner it will become second nature to you.

In this particular case I think it's usually easier, more efficient motion to use your middle and index finger on the A & D strings.  Then moving from the E minor chord to the A minor chord, you keep those fingers together & simply shift them each over a string to the D & G strings.  Your index finger was completely free to hover in place until needed on the B string.  Using your index finger on the A string means you have to move all your fingers around more than absolutely necessary to get from E min to A min.

As long as your fingering works for you, and your can play the chords cleanly, in time, that's fine.  But I encourage you to work on that different fingering as well.  

Hope that helps!