"Sweeter" is also a subjective term. They might have the same amount of sugar, but Pepsi definitely tastes sweeter than Coke when just opened.
To me, for lack of a better word, Coke has "harsher" bubbles that somehow drown out the sweetness. Which is why a can of Coke that's been open for an hour is way too sweet to me.
The bite from the carbonation is the flavor of carbonic acid, which is unstable and will resolve back to H2O and CO2 when not under pressure. A heavier carbonation ends up having a higher concentration of carbonic acid when you first open it up, but as the bubbles burst the concentration goes down.
To me, for lack of a better word, Coke has "harsher" bubbles that somehow drown out the sweetness. Which is why a can of Coke that's been open for an hour is way too sweet to me.