I like them both.. However, the OP fails to bring up Demi's second single, Made In The USA, which completely bombed despite a handful of performance spots & a 69-cent discount corresponding with a Teen Choice Awards performance. Selena's second single, Slow Down, was a moderate hit despite suffering a music video leak 2 months before it was official serviced as a single.
It's obvious that it was Selena's choice not to continue with the era. HWR gave Demi a third single after a failure so there's no doubt they would've released a third single for Selena after Slow Down being quite successful. DEMI is 4 singles in & was released 2 months before SD so it obviously should be outselling SD. Demi also had X-factor as a large platform to keep her in the spotlight & is about to embark on a second tour supporting the DEMI album.
To answer the question, I don't think Demi is as big of a star as Selena yet. Maybe if this poll was based on Brazil & South America, I'd go with yes.