Just because they've both been gone 2 years doesn't mean it's a comeback. Britney did her tour then the Xfactor
Beyonce is currently on tour now. A comeback Is coming back after being out of the public eye for a few years and not doing tv shows or tours
Beyonce's "comeback" would've been insanely huge if she used the Super Bowl to her advantage as she should've. Now the hype is gone, so Britney's is probably gonna be bigger.
Britney, as it should be. I could see Beyoncé taking album sales thanks to her years of promo and the multiple editions she puts out for every album, but with single sales and radio play, Britney should be able to breeze past her for the bigger era