What time of the day will you be here? That does make a bit of a difference, because if it's a night-time stopover it doesn't really make a lot of sense to leave the airport. Everything will be closed anyway.
If it's during the day, ie anytime between 10 AM and 10 PM I'd agree with the Festival City suggestion. Dubai Mall is a bit risky, because first of all the distance to the airport is a bit more than that of Festival City. And there's a lot of construction going on in the Dubai Mall area, so you can easily get stuck in traffic there.