In order to assist you in making the correct choice of instrument please take some time in analysing your couplers/registers of both the bass and treble sides. Some of your registers are called "transposing couplers". Some will give you different pitches and/or combinations of pitches (see diagram). In order for you to play a certain musical phrase/passage in your piece, you might need to apply a register in order to obtain a true pitch of what has been written, or simply have the right "sound colour" for the passage in the piece that you are playing.

Please note that usually block reeds in tone chamber are marked with an empty circle on the register, and reeds out of tone chamber are marked with a filled circle on the register. Also, not all accordions have a tone chamber called "cassotto".