During the film days I had quite a few filters of different types which I eventually adapted to a Cokin P holder. I even have a homemade matte box that will work with the Cokin holder.
Now that my film cameras have been sold (except the 4x5) and everything is digital, I am convinced that all I need is a CPL and a couple of NDs. I'm still carrying a grad but I think PS and LR are usually better and more flexible choices. But you have to shoot RAW if you don't use Grads.
So I have simplified my system to 77mm round glass filters and step-down rings. I have a CPL, two NDs and a much lighter load. No more square filters, holders, adapter rings or their expense. And I am concentrating on the photographs instead of the stuff.
Having said all of that, I think that, if you're going to use squares, plastic ones (or at least popularly priced plastic ones) are not as good as glass squares. The optical resin squares that I have used have had bad color shifts and degraded IQ. I think Cokin still calls them "organic glass". Well,if you remember your HS science class, organic just means carbon based. It's plastic. I know it's supposed to be optical quality but it just doesn't seem to work as well as good optical glass. I think "Hitech" may be worth looking at but that's about all the plastic I would consider.
The Lee holder setup is pretty pricey. About $200, I think. There's another reason to consider round glass filters with step down rings and software solutions for the grads. FWIW.