@ben, are you sure you can't keep them both? Personally, I would try find something else to sell if I needed to. How else is your accordion collection going to grow?
Seriously though, I think it's fun to have the option of different sounds for different styles of music, and I think it's possible to love more than one accordion. You didn't say if you enjoyed playing one instrument more than the other. My original accordion is a Marotta Concerto of a very similar vintage, and, although dry tuned and only LM, it just plays so smoothly and nicely that I still turn to it often, particularly for lessons and when I'm learning songs--even those I'll probably want to eventually play on another accordion with a wetter sound.
Just looked at the picture that shows the register switches--are you sure the Concerto is completely dry tuned? I can't imagine why there would be a musette option with two M reed banks that didn't have at least a little difference between them. Perhaps it just sounds dry compared to the other?