Its really all debatory, individual plants within a species can vary in alkaliod diversity and quantity. Echinopsis Peruvianus does have more variety of alkaliods but most will want high mescaline so other species such as echinopsis langiformis (formaly trichocereus bridgesii) echinopsis scopulicola, peruvianus and pachanoi all have similar mescaline content. There us a good overview of psychoactive cacti on the cactusmall website deffinately worth a read.