Beyonce is hollow when she sings to be honest. It's technically perfect but it's hollow and clean like a whistle. There is no emotion to be found in her vocals. She is a lot like Leona Lewis in that way. Rihanna has a tone that is more conducive to emoting. Just listening to the "we were always just that close" line of California King Bed shows more emotion then the verses of 1+1 which should literally be dripping with emotion and they just aren't. It's all detached and cold proficiency. Beyonce is the better singer, but not the better emoter.
... this is one of those things I feel like shouldn't even be a question.
IMO Rihanna uses the same dynamics in every song. When she trys to convey different emotions they all tend to sound the same. Passionate= screaming, Happy= screaming, angry= screaming, sad=muffled, seductive=muffled