As someone who teaches a bunch of swing and other dances, competitive and social, the 2 styles' differences probably come down to context. When I go to a lindy or wcs party, I'll do 1 dance all night to a variety of music. In this situation the dance tends to be relaxed with casual styling, and lots of room for improvisation. That 1 swing dance has to be sweet, lyrical, fierce, playful, smooth, punchy, or sexy as the songs change.
When I got to a ballroom, country, or california mix dance, there's usually 1 or 2 dances which are developed and rich, and 6 or 8 other dances in which people know 3 moves.
They have a different dance for each emotion- the angry tango, the cheeky cha cha, the bouncy swing. The range of expression thus gets spread over several forms, which then become stereotyped. There's nothing more inherently sexy about qqs, but rumba is the one always done 'sexily' so everything is styled to enhance that.
Thus a 'Ballroom' swing is restricted to one emotion, and the styling is tweaked out to emphasize a certain look, with the other stylings reserved for other dances.
Also, the ballroom community groups swing with the Rhythm, or Latin dances. This means that the movement technique is more like cha cha and rumba- cuban/latin motion applies.
This is not true of the swing dances, be they on bent or straight legs.
Do charleston(from which swing descended) on the followers foot (back rock on right foot) and salsa/mambo. They have the same basic, but look and feel totally different. Charleston (and swing) swings the hips and legs over multiple steps, while salsa shifts the rib/hip relationship side to side on each step. Thus during the 8 beats/6 steps a charleston dancer will have 4 hip actions, while the street salsa dancer, or trained mambo dancer will have 6.
Social latin movement has been refined into cuban walks and latin walks, whereas the vernacular jazz age dancing (shorty george/boogie walks, duck walks, fish tail) is more like a modern runway model's walk- hips first, not chest.
Ballroom swing briefly incorporates this movement as chicken walks, but for the most part, it's a Rhythm dance.
WCS does cuban movement (digs and press lines) when grooving in place, but when traveling down the slot, it's more like a ballroom 'Swing' dance (foxtrot and quickstep were begun with the same music as lindy) with 1 hip action for 3 steps. That's why swing teachers often say that we don't use our hips, when in fact we do- just not on every step like salsa or cha cha.