Hard choice. Piano would easily get you a decent 6 figure salary, and you could probably build up a bit of fame.
Singing...it takes more than just a good voice to make your career in that. Does this skill include composing songs and writing lyrics? If so, then singing. If not, then I'd rather live the life of a simple pianist than deal with record companies practically owning me.
"If history is to change, let it change! If the world is to be destroyed, so be it! If my fate is to be destroyed...I must simply laugh!!."- Magus