^Yeah, the rims and dome are the major differences.
Riots have the most slanted plus ( + ) rim i've ever seen. And the vast majority of optos are flat as a pancake. So they are easy to get up to speed, hold very straight low lines, and fade out hard at the end... but you have to play with your grip to get good friction.
Flows don't have the plus rim and feels like gripping a wider rim striker. However their dome varies greatly. they have much more glide and high speed turn.
Both are good discs.. they don't overlap at all imho. I had a first run opto riot that i loved for low lines. I had beat it in pretty good over 6 months and had it perfect. Then i lost it and had no patience to beat in another opto (gold lines were not available at the time).