"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.
While it differs for different cultivars, I don't think "a lot more" is true in general. For the best tasting commercial varieties, I think they are roughly comparable.
Hayward Kiwi (the most common commercial variety in the US) are harvested fairly hard at about 6 Brix (approximately percent dissolved solids assuming those solids are sucrose) and ripen up to about 12-14 Brix.[1]
Strawberries are usually 8-10 Brix, but a peak of the season fully ripe (and delicious) modern variety like Seascape or Tristar might be 12-14 Brix[2], the same as a fully ripe kiwifruit.
I suppose it depends strongly on the particular cultivar. I know I was surprised by the difference when I looked it up a couple of years ago. I was doing a keto diet at the time, so you read a lot of nutrient tables.
