I use Lee filters as I believe the are proper neutral density rather than grey which the Cokin P series are. I actually have both systems and find that the Cokins scratch easily.
I use a Cokin X pro on a Sigma 12-24 with a full frame camera. Vignetting to be expected below 16mm.
If you want a polarizer on very wide angle you will probable need to go 105mm like I did. (pricey)
All in all the best value is definitely Cokin P series. Lee maybe best quality.