@rich3 I can play in Em on a D harp in 3rd position, on a C harp in 5th position, a G harp in 4th position, and an A harp in 2nd position and be perfectly happy.
If the tune has an A7 (as opposed to Am7) then I might choose the D harp in 3rd position (this is what’s referred to as the Dorian mode, which I explain a bit here.)
But none of those options give me the CHORDAL options that I have on a Natural Minor harmonica. Playing a Natural Minor harmonica is nice for a beginner or intermediate player (and for advanced players too) because whatever bag of tricks you have for 2nd position carries over perfectly, it just sounds minor.
The only thing to note, in our example here, if you blow on an Em you get an Am. So the Natural Minor harmonicas are great fro songs that are in, err Natural Minor. If you’re playing Dorian which has the A7 instead of Am7, you can still use a Natural minor, you just have to avoid 2 and 5 and instead play -2’‘’ and -5’’ over the IV chord.
I’m sure I’ve now rambled way more info than anyone cares to read. 