I'm going with Selena. Selena has a #1 album worldwide and Demi doesn't. All 4 of Demi's albums combined is still less than just 3 of Selena's albums combined. Selena has always sold a mere consistent 500k+ albums outside the states with each album and Stars Dance debuted 2x more than DEMI did outside the states. Selena's "FLOP" album sold at the least 900k WW while Demi's best sold 1M. DEMI only sold less than 100k more than SD in the U.S. and WW because she had 2x the singles, tours and her era lasted twice as long and even then, DEMI has yet to reach gold status in actual sales. Overall Selena will beat Demi in the U.S. as per usual but WW, it could be either or.
And not lovatics coming for Selena's promo for Come & Get It when Demi's single GYHAB had 10+ televised performances, more than Selena ever has