The plastic and gelatin filters that I use in my Cokin filter holder meet most of my needs. However, if I needed high quality graduated neutral density filters because landscape photography was my passion, I would get a Lee filter.
I have a Cokin system (sorry I had to use the system word), and yes *most* of them are plastic/resin. The polarizers (cir * lin) are glass and I have not noticed any unusual cast in the ND filters.
It's not so much settling as I have not noticed anything bad to warrant getting the Lee.