I am looking to buy some filters

I use a canon 450D with 3 lenses - canon 18-55mm, tamron 70-300mm and a tamron 90mm f2.8

The first filter I would like to get is a polarising filter and then i would like to get some ND filters

Now I am not sure whether to get a screw on polariser or to use a square filter system

From the point of view of the neutral density filters I think (from my understanding) a square filter system would work best as it would give me more flexibility as you can slide the filter up or down depending on where you want your horizon and how much you want the filter to affect the image - please correct me if my understanding is wrong

However do I get a polariser to go with the square filter system or go for a screw on - I did read somewhere that a screw on would be better

If I was to go for the screw on my plan would be to buy for my biggest diameter lens (62mm) and get step-down rings so it will fit my other lenses (a 55mm and a 58mm)

Also this all brings me to my next problem - what make to get

There are plenty of very cheap polarisers on ebay but I dont want to get a cheap one that will do the job poorly (I am trying to improve my photography), I have seen a few places on the web that suggest Hoya is a good make, could anyone give me some help with this? What about Kood, Jessops...

With regards to a square filter system - is Cokin all that bad? I have heard some people say not to use them but their prices are good - but does this reflect the quality?

Any advice is really appreciated