One can play high C in first position with only a slight stretch of the pinkie. It is commonly done. Also, for just the C, one would normally use second or half-second position instead of third.

Shifting to E or D allows the use of open strings for drone or unison notes, very characteristic in fiddle improvisation. Stewart is right that the flat keys have a different sound than the sharp keys, and transposing will alter the mood of the piece somewhat. Whether this is sufficient reason to give up the obvious advantages of the more common "sharp" keys is for each player to decide.

Personally, I like it when players vary the keys they play in. No matter how accomplished the player, it gets deathly dull hearing (or playing) piece after piece in the same two or three keys, and only in major or minor. So heck yeah, learn the piece in Eb--what a treat!