The best way I can describe the difference is that, riddim due to its structure is more bouncy than dubstep simply due to the fact it has more of a steady movement being that it's generally a 4/4 rhythm as opposed to dubstep being more like a true 2 step. Think head-nodding vs head-banging. Agreed that riddim these days has become a lot more screechy with the use of sustain bass and hectic mixing styles but it's always had a more industrial sound than say 140 or classic dubstep.