A friend and I recently came up with the phrase 'hedonist generalist' which allows one to pursue and learn things, not because one 'needs' the information and skills, but because the pursuit and the learning feel soooo good.
As for ease -- I've played the fiddle about 8 times longer than the concertina so it feels easier, or at least more intuitive, but I've progressed much faster on the Concertina than I ever did with bow and strings. Whether this is due to the relative ease of the instruments or because I came to the concertina already reading music (after a fashion), and with tunes in my head, I couldn't say.
Certainly one of the joys of playing both (at least for me) is that some tunes (though it is probably my fingers) seem to prefer one instrument over the other. When some sequence of notes just baffles me on one, often as not it'll come dancing out on the other. Another of the joys that the concertina has helped to fine tune my intonation. I get sloppy when playing by myself, flats and sharps not quite what they should be, but when I get to know a tune thoroughly on the concertina, I am less likely to be satisfied with almost-but-not-quite-in-tune with the strings. And since one of my goals is to 'play well with others', decent intonation is a must.
And yes Rick, I, too adore my fiddle. Would that I will be a fair hand with both, some day.
Sarah