Sweet onions may have a bit more sugar than regular onions, but they don't always, and they've sometimes got less. What sweet onions have is an absence (or lower levels of) of the sulphur compounds that make onions (and alliums in general) pungent. That's largely a function of where they're grown, which is why the best known variaties (Vidalia, Walla Walla, Maui) are named after particular places.
The sulphur compounds are broken down during cooking. The only thing you'll gain from them is less irritation when cutting them up, which might be enough reason to use them. It's not for me, but I'm cheap.