Although Gaga is by no means some sort of vocal prodigy she's obviously a better singer than Rihanna. But with that avatar I didn't expect any better from you.
Rihanna. She has a more distinctive, interesting voice and her vocals are improving year by year.
Gaga has a powerful voice, but unlike Rihanna's it is not versatile and only suits a certain type of song, hence why she hasn't recorded a ballad yet.
It's the complete opposite.
Rihanna's voice sounds good only on certain songs while Gaga can sing any and every song out in front of her and she doesn't have to stand still to do it.
Of course it's Gaga. Rih finally gave a pretty good vocal performance at the Grammys though.