She would do both, of course. I'd open with VOL (if not only for the ominous sound at the beginning
) and end with "Hero" with a little uptempo, Make It Happen, Heartbreaker Love Hangover, and Always Be My Baby slayage in the middle. I had a whole list in my thread last year
The only thing is Mariah sucks at medleys... I'd rather she do large chunks than anything too creative 
She'd have 13 minutes, so...
3 minutes VOL
2 minutes Make It Happen
2 minutes Heartbreaker (Love Hangover remix) - Bring out Diana? C'mon for THAT slay.
3 minutes Always Be My Baby
3 minutes Hero