If I were you, I would go with either Step You or Moments. Greatful Days sounds a bit annoying to many people and Heaven sounds boring in the beginning (I wouldn't really sing a sad song either).
Which song you would like to sing? Step You seems like more of a crowd-pleaser with its upbeat tempo, but Moments is such a passionate song.
Also take into consideration your vocal capabilities. If you think that you can hit all the notes in Moments, then go for it.
Otherwise, go with Step You, which seems to be the more obvious choice. It's such a fun song to sing, the chorus is easy to learn, and it isn't so hard to sing either.
