The only thing with polystyrene incubators is that they are difficult to sterilize after a hatch. They are also easily damaged, and can start to look tatty after a season or two.
Having siad that polystyrene incubators are not that common in Europe, so I don't have that much experience with them!
I currently have 4 plastic walled, forced air incubators - ranging from a very basic 60 egg manual, to a 24 egg fully auto. My to fully auto incys are ady by RCOM, and although expensive, worth every penny. Temperature and humidity are auto controlled, and both are forced air, so the temp and humidity are equal throughout the inside.
I know there are lots of debates of still air vs forced air, but forced gets my vote every time (much easier to get the temp and humidity back up when you take the top off!)
But I suppose it depends on what you will use your incy for, and what kind of budget you have!
Good luck with your decision, and happy hatching!