| I don't think sending in an arts supplement ever hurts. The admissions people send all the arts supplements over to the music department, and the music department returns them a list of the top few, who get some minor boost in the admissions process. Everyone else is just considered normally, so there's no penalty for giving it a try.
For what it's worth, I submitted a music supplement with all my applications, and heard firsthand (from the music people who called and emailed to try to get me to come to their schools) that it gave me a boost at Amherst and Middlebury. I'm not sure if it helped at Williams since no one ever contacted me about it - Williams overall does much less to reach out to admitted students - but I would imagine the standards are similar. For reference, I was an all-state level player (maybe nationally the 200th best player of my instrument in my year, to make a completely wild guess), but certainly not a virtuoso.
Music at Williams is good in some ways and bad in others. There are occasionally decent concerts (depending on what you like), but if you want an exciting local music scene Amherst is definitely the better bet. Anyway, I wouldn't worry too much about the differences until you know you've gotten into both - there's a decent chance your decision will be made for you. | 
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