The metal body is more for feel than anything else* - they introduced it for people who wanted more heft in an Aztek, so the weight is more like ABs of traditional design.
Personally I like that the plastic/resin-bodied ones are so light, especially when I'm spraying for a long while, but other people have issues with the low weight.
*Obviously aesthetics come into it too, no denying they look nice.
Einion
This was what I figured. As I´ve never held a airbrush in my life I won´t be biased against the lighter plastic ones, so I´ll probably end up getting one of those.