Every time I go want to go a festival (with sessions) under my own steam -I don't drive would have to take train which, means using the underground-I'm faced with the same dilemma. I have a concertina and a melodeon. I don't have room to take both.
One side of me thinks take the concertina its smaller, lighter and more portable. But I find it hard to hear myself in sessions playing it, and struggle to keep up. Plus I'm not confident picking up songs by ear on it.
The other side wants to go for the melodeon. I find it much more comfortable to play, I can keep up and even have a go at trying to pick out tunes I know without dots. But I carry it in a backpack which means I can't pack essentials in a backpack and I don't fancy dragging suitcase up in down stairs/escalators.
I have recently purchased and begun learning fiddle. So now have another instrument to wonder about bringing.

What would you choose?