Hmm... After sampling both artists, I believe I can give a summary of my (non-expert) opinon of each.
Gershiwn: Elegant, loud, and powerful. He has a good flow from point to point, and clearly it was difficult to pump out 16 minutes of sheet music a song. However, like any artist, his works become repetitive, this is not helped my the lack of lyrics (though to be fair, lyrics t=would ruin it). Overall, a very good artist who deserves to be on the list.
Holly: His music, makes me want to dance. It infiltrates me and infects me with an amazingly...swing-y? feeling. Not to mention he died young (like many good artists), and has numerous songs devoted to him. I do indeed like his music more than Gershiwn's and combined with his influence on later rock and roll.....the choice is simply.
My vote goes to Buddy Holly.