It looks like there are a few versions of the collections but Origins is the one that is ongoing so if you intend to one day own all of them and want them all to match, that's your best bet, though you might find the older ones cheap on ebay or something. Normally the collections will have information on what comics they reprint. There was only one main Spawn volume (called just Spawn), so if the collection contains Spawn #20-30, then that's #20-30 of the regular series, so you can put it together that way if you mix and match.
I LOVE BLUD BLOOD! - Rob Liefeld