I want to throw a tidbit into this discussion...Get This!
I listened to both songs and in my opinion...
"All My Life" would have been perfect for Michael...along with brother Jermaine or Randy harmonizing with him...
Are you with me guys?
KC & JoJo have such great harmony as brothers with their voices, so it would make sense if Michael would sing it with a brother, also.
Now, the other song, "Life", I don't see that as a Michael Jackson song, only because of the arrangement for KC & JoJo. The music would have to be very different in that song for it to sound as soulful with Michael's voice as it did with KC & JoJo.
Again, the harmony works with KC & JoJo, instead of one single vocalist, although Michael has a way of putting harmony into his background sound.
But, "All My Life" would be the better song out of the 2 for me to choose for Michael to sing.
And..."You Are Not Alone" sounds great when you really listen to feel what he's singing about.
You have to meet him there in the song and feel its dynamics. I feel it every time I listen to it.