Also, think carefully about what lenses you're going to use, both now and potentially in the future, as to what size filters you'd want.
I've personally got Cokin A and Cokin Z.
A fits up to 58mm or even 62mm-threaded lenses, ie a whole lot of my older and slower 135 SLR gear, and some of my smaller LF lenses. They're also cheap as piss, you can get almost any of them for $3 each.
P doesn't really fit with what I've got, only my longer 135 SLR gear and standard-normals on MF (neither of which I'd normally use filters on).
My wider (landscape) MF lenses have up to 86mm threads, as do my bigger LF lenses (90/5.6 SWD). For them it's the Cokin Z, 100mm wide filters, also compatible with the Lee 100mm filters so you can mix-n-match.
I'm almost regretting not springing for the 150mm Cokin X because the centre filter for my SWD has 105mm front threads, so I can't use ND Grads and centre-filter at the same time. But I can easily justify going for only 100mm when I see the price of the 150mm wide filters.