I prefer Glory but both are good.
As far as album sellers goes there is no competition (like it's 84M vs 34M people
), Britney is the 5th best selling female in the US, 8th WW, so saying Britney is the better album seller is a simple fact, she's one of the best EVER.
Britney is the 6th best selling female WW (albums + singles), she doesn't have the same PR machine as other artist may have, but I think some of y'all should learn to respect ha name.