To get back to the question of what's a good key:

1. If it's a vocal piece, pay attention to the range, not the key. The highest note should be no higher than the D which is the second line of the staff. The lowest note should probably be no lower than the B below middle C. (for sopranos' sake)

2. I have small hands, and I find C hard to play and F impossible. I much prefer D,G, and A chords and their relatives.

3. I once told a guitar teacher (an expert one) that I had trouble with C, and he said, "You're not supposed to play C on the guitar."

4. Many, many traditional dance tunes are in G and D already, which means that others have memorized them that way. So go with the flow.