If you've been keeping up with the producers that are working on Britney's 8th album you'd already get the sense that it will be a pretty solid album. (Lets just say she isn't using the Dr.Lucks anymore)
Gaga has Zedd who is pretty new but nothing revolutionary. DJ shadow isn't the best, etc.
Overall Britney's album will probably be better than Artpop.
Gaga will probably promote a whole lot more. However she lost a lot of the general public, promotion didn't help EOG, Judas, Y&I, or Marry the night.
If Britney promotes, she can slay everyone in the game.
Pretty sure the vegas r will generate a lot of attention for album 8.(promotion may go hand in hand).
Will he a close battle.