Awards doesn't mean "Critically acclaim", awards are just a celebration of the own bussiness and most of them (like the 95% of them) are based on popularity.
Critically acclaim is perceived on official magazines or by proffesional music critics, Britney's performed slightly better than Mariah in that Area , most of britney albums got better professional reviews than Mariah's.
Mariah is praised as a great vocalist all the way though.