What do you prefer, grapes or cherries? Perhaps this is not acceptable because it specifies a person?
Yeah, the given sentence asks the question generally. If you want to direct it to a person it would be :
"מה אתה מעדיף/את מעדיפה, ענבים או דובדבנים?"
That would be "מה מועדף". The word עדיף suggests it has some kind of advantage, that it's better - it doesn't mean it's preferred.