CS is for low end PCs. They aren't made as well as RM PSUs but they're still decent.
RM is made to be silent however some of them can get hot because of it. The fan doesn't spin under about 50% load iirc.
CS series is more for budget users and more affordable. But just because they're budget power supplies does not mean they're not good enough. CS series offer 90% - 95% efficiency.
RM series on the other leans on quiet performance. Utilizing that plus the fully modular feature of their power supply is a good buy. They're usually 97%-100% power conversion efficient.