I just wanted to add that playing in key should also be a consideration. There are players out there that won't shift things up or down a key to accomadate a 'D' whistle or a 'G' harmonica. There are a few common keys that most people play in, but sometimes people don't know what key their playing in. In these cases, you have to watch and listen in order to figure it out. I have a particularly hard time doing this with fiddle. I guess the notes are too high.

It sucks to hear someone playing his 'D' whistle, when everyone else is playing in 'C', because he doesn't know. It's even worse when their ear isn't developed enough to tell them to stop playing.

My vote goes to the guitar. You can usually get away with only knowing six chords, and there's almost always someone there to follow.

--CS