Don't waste your time trawling the internet for side by side comparisons, just buy a set of cheap filters and see for yourself. If you are not happy with the quality they give you, spend the money on upgrading. Cheap filters are too cheap to wring your hands over!
well even a set of cheap cokins (with holder and adapters is 100+ euro) ... and im a student i have to hold my money together.
thought if cokins are really bad and singh rays would last me a lifetime i would spend the money. singh rays are scratch resistant are they?
All resin filters are going to get scratched over time; more expensive filters may last longer, but I would not count on them to "last a lifetime". If you're hard up then don't blow your entire budget on filters, just get some cheaper ones and live with the compromises they entail (colour casts -especially when double stacked). I would start with two ND grad filters: a 2-stop and a 3-stop (don't bother shelling out for 1-stop, unless it's part of a set); a holder and adaptor rings. I would choose soft nd grads, as they are more versatile (the exception to this is if you are using 4"x6" type, e.g. Lee's, on APS-C -Lee recommend hard ND grads in this situation).
When chosing a holder, there can be a trade off between versatility and vignetting (and I mean very hard mechnical vignetting). I found that the standard Cokin P-type holder vignetted with the Sigma 10-20mm f/4.5-5.6 below 12mm. In this situation, I had to use the wide angle (single slot) holder instead; although you could try modifying the standard holder with a razor saw. This may vary with different lenses, so I would try the standard holder first.
If you must have the best, then the Lee system that I have upgraded too is fantastic in both performance and price (£35 for a 77mm wide angle adaptor ring! It's just a piece of metal for crying out loud!). B+W make glass ND grads for the Lee system, available from B&H for $270 each...