These are two of the main popstars (minus Beyonce) who execute choreography routinely during performances. Who is the better dancer of the two? Let's converse and keep it cute.
Gaga by far, because she actually sings while she dances, too. Rihanna's choreography usually involves her putting the mic away from her face because she has to concentrate. Gaga is able to execute the choreo while still singing the song. Sure, sometimes her outfits get in the way so she looks a bit stiff every now and then, but in general, she gives it her all, while Rihanna half-asses it most of the time.
Neither of them are dancers. GaGa has had great choreo designed for her where the moves are more eccentric/artistic and fits in with her lack of fluidity. It is easily replicable by anyone who practices for a bit. I personally enjoy both but there needs to be a certain amount of swag to be able to execute the choreo the way Rihanna does.
Rihanna has only recently started to do choreo but based on her songs, it only made sense for her to dance during the breakdowns. So for those people saying Rihanna can't dance and sing simultaneously, we haven't had proof of that yet.