Matter of preference really on one hand and price on the other hand. A good quality airbrush and compressor you are talking in the 250-300 area and you haven't bought paint and containers yet.
Powder painting costs vastly less to get started. Can spray paint with powder paint also with a lot smaller investment. Compressor and spray gun easily under a 100. If you study a lot of the paint jobs on here. I would bet 90% of them are of the powder paint variety. The myriad of special effect is just as achievable with powder paint.
The possible exception would be a scale effect that an airbrush achieves easily. I have yet to find a way to do it with PP. But someone brighter than me will figure it out shortly.
Since you already have the equipment the start up price does not mean much to you. But it does to many beginners. The learning curve on PP is a lot shorter and easier than airbrushing also. A fluid bed helps but is not absolutely necessary. Basically a heat source , a jar of paint , a toaster oven and container of paint. So the simplicity is quite attractive to all.
While comparative costly at first . More and different sources have contributed to the price falling. Making it even more attractive.
