they arent, really lol. just different. infact most pcie cards are just pci cards with a pci to pci-e bridge chip on board. if you're intending to by a card that comes in both variants, just choose the cheaper one or the one that gives you the most flexibility. ie, some people are better off going pci-e if they have all their pci slots occupied, or some might not be able to go pci-e because their pci-e slots are occupied are cant be used due to obstructions from other card ect