I say buy SC1 and CT but if you really want to know the story very well then buy SC1 and PT then buy CT and DA xbox or if you want the DA 360 but the xbox version is better IMO.
Well I used to have all the SC's for PS2 but my PS2 broke and I got 360 so I'm going to buy ALL SC's for Xbox, so I know how the story in SC goes but, yeah SC1 and CT were my fav SC's.