Wishes is quintessential Disney. If I had to choose one or the other, I'd pick Wishes.
Symphony in the Stars is outstanding. It's cool and worth seeing but, to me, not if you had to choose.
I'd always pick seeing fireworks over Cinderella Castle than the Chinese Theater.
I see Symphony in the Stars as a bonus but not my one and only fireworks show. This is purely my thoughts on the show, not dessert party.
Will they be able to see Wishes on your trip (in other words, are you picking one dessert party but still seeing the other show on another night?) If so, the Star Wars dessert party sounds cool, so you might want to do that one.
It also depends on if they've seen Wishes before or if you go back often. I'd consider going to Symphony over Wishes only if they are Star Wars super fans and they've seen Wishes before, or will be able to see it another time in the near future.