I think when combined with DC, there is a case to be made that Beyonce was bigger than Eminem in the 2000s, in the US at least. Beyonce was the top RIAA certified artist of the 2000s in the US.

Beyonce Tops Decade's RIAA Sales (billboard.com)

But while Eminem was the top Billboard artist of the 2000s, Beyonce was the top female Billboard artist of the 2000s (she was 4th overall). However, DC was also 9th best Billboard sellers of the 2000s, so there's the very real possibility that combining DC with Beyonce's sales, would put Beyonce ahead of Eminem in the USA.

Artists of the Decade (billboard.com)