I think the large sq ones give you more flexibility. They can be moved/slid in the `holder` for position needed & easier to stack more than 1 if needed.
Lee filters are reckoned to be amongst the best I think. http://www.leefilters.com/camera/
Suppose it's like most things, you get what you pay for.
Also, you'd only need to buy an adaptor ring for different size lens threads.
btw,He who dies with the most toys, WINS!Nikon D800E & D700 bodies + Nikon 200-400mm F4 VR1, 50mm F1.4G, 16-35mm f/4G VR, 105 F2.8 VR macro, 70-300mm lenses. A couple of filters, Giotto tripod & ballhead. Lowepro Slingshot 302 AW