the blizzard is the better card. Its much better in construction. It can take more memory (256mb in total vs 64 on the viper). It also can have DMA scsi with the addition of the scsi board.
Unfortunatly I dont think you can buy them new anywhere anymore, at least not at any of the well known dealers... perhaps someone in the US has some, or that italian shop but I dunno...
the only real plus I can think of for the viper is that its avalaible new, and you can clock them higher than a blizzard card.
