Rihanna is much bigger in Europe. Gaga's last hit was Telephone here.
That's why Alejandro, BTW, Judas, Edge of Glory, Applause and DWUW all went Top 10? And why Gaga sold out of all her UK dates within 5 minutes and added 5 more dates? Ok sis
R8's current state balances the odds tremendously..
It's shaping up to be a ARTPOP sized mess, if not bigger, commercially (Artpop's singles have better stats than R8 so far)..however Rihanna's celebrity status/relevancy remains intact, while Gaga's has decreased considerably..
On the other hand, Gaga's album sales are more impressive, she's more impactful in a sense (due to her iconic peak), is more acclaimed, better touring reciepts, and her "brand" is bigger.