This works a lot of Air Balloon Heatran, meaning it can wall stuff like Garchomp really well. However, Air Balloon Magnezone is only really needed when you have a really big ground weakness on your team, otherwise this item is pointless.
Gives Magnezone a ground immunity
Makes Magnezone only have two weaknesses (Fighting and Fire)
Allows it to wall ground types pretty well
Cannot come in on an attack, for fear of breaking the balloon
Only one time use
Leftovers, in my opinion, is the best item for Magnezone, because it allows recovery. However, only have Leftovers when you have a ground resist or immunity on your team, as so you don't open up a ground sweep for your opponent.
Gives it recovery
Can wall stuff better
Gives it crippling 4x ground weakness
Overall, it mostly depends on how well your team is synergyzed, but if you ask me, I would say Leftovers is the superior item.