I'm very new to the world of miniatures, but I have to say I prefer painting the metal figures to the plastic ones. Metal figures have a satisfying weight to them, and the density of pewter feels sturdier in my hands. I also feel that they take to paint better than plastic figures (although I use a brush, not an airbrush).
That having been said, I do see the merits of plastic figures. The detail, customization potential, and lower cost are all wonderful traits. Looking at the new Malifaux figures, I am wowed by the attention to detail that the new plastic figures show (currently painting the Dark Debts crew).
I'll still take a pewter figure over a plastic one any day of the week, but I can appreciate the beauty of a well-done plastic figure.