Both kinds of anti-spasmodics have the effect of blocking acetylcholine receptors in smooth muscle cells (the kind found in the gut). The difference may simply be in which receptors the drugs act on. For instance, there are several different SSRI (selective serotonin reuptake inhibitor) medications used to treat depression, and they differ simply in the receptors that they act on in the brain. I think the same is true for hyoscyamine and dicyclomine. What works well for one person may not work as well in another. Fundamentally, they have similar actions.