Neither have strong voices, but I do think both have talent. Rihanna has such a flair for performance, a big stage personality and the album she had the biggest hand in creating - Rated R, was easily her best. When she has the right song too her voice works - the emotionless, icy thing she has going on sounds fantastic.
Cheryl as a part of Girls Aloud worked really well - she was great at harmonizing, and made the most of her solo slots. While her voice isn't powerful, she has such a nice tone to it and a great talent as an on-screen presence, clearly, as she's largely built her solo career off her personality.
So I'd probably say neither of them will ever trouble the likes of Adele voicewise, they both have something special.