The Chemnitzer is more like a bandoneon, quite large, square, used as lead in polka bands. Multiole reeds per key, so it sounds more like an accordion. I think of it only as a concertina in that both left and right hands have single notes, (no chord buttons), which to me has always been the defining difference between a concertina and am accordian.

But no-one has said much about the duet. Someone loaned me a Crane duet once, and I couldn't figure it out at all. I once heard that Tim Laycock learned on a Triumph system duet because that's the only concertina he could find that was affordable. Basically, the Duet is a chromatic instrument, with push/pull on a button giving you the same note both ways (like the English), the main difference being that you have all the notes on each hand, rather than split across the two sides of the instrument. The left hand is pitched lower than the right hand, though there is some overlap. The (very basic) idea is that you can play the melody on the right hand and an accompaniment on the left, like on the Anglo, but you're not limited to the diatonic push/pull availability of notes in a particular direction, and you're not stuck in one key. There are several systems, McCann is perhaps the best known. One that's become more popular in recent years is the Hayden, which has the advantage that to play in a different key you just start at a different place; the fingering pattern is the same. One of the musicians on our local morris team uses the Hayden quite successfully.

Noreen, you have to give some idea of what style of music you like to play to get a good answer to your question. You could play "This Land Is Your Land" quite successfully on the concertina, but that may not be what you have in mind.