The Metropole has an amazing French patisserie (for SE Asia) on the ground floor. There are also some very good restaurant's nearby.
I've not stayed at either, though I've enjoyed the restaurants.
As busylizzy indicates, few Travelfish contributors would seek accommodation in that price range.
I've inspected both, stayed in neither, but can say, as long as you're housed in the old (original) wing, the Sofitel is easily the most fabulous place to stay in Hanoi and the Sheraton isn't even on the same page.
However, if you're in the new wing at the Sofitel, there's little to separate the two.