It's a choice. Either chord could end a piece (and there are some examples with the sampe phrase ending differently.) In some modes, the ending chord was always major; sometimes this practice would carry over to key-centered music.
I've found that some tunes I've written sound better with a minor chord ending and other actually sound better with the Picardy Third. I don't know why so I just try both and see which I like better.
On a similar note, (so to speak) in one of Bukholfzer's books, there is an arrangement of Greensleeves which uses minor v chords (and minor i) when a section has more following and a V chord on the final cadence (V-i). Sometimes it's just taste.
In playing strophic music (like hymns or songs) in a minor key, it's sometimes useful to play a V-i cadence at the end of each verse and a V-I to end the last verse; this signals the audience that something different has happened. (I might even use v-i internally, V-i at section ends, and V-I to end the complete piece. There's not much harmonic change but listeners should hear the section markers.)