Are you serious? There is no way Smash 3DS "equals Brawl's amount of content". lol
This is ignoring Subspace Emissary, mind you. Otherwise, duh, Brawl has much more. This is how I see it:
Smash3DS has more than Brawl in every in-game category: characters, items, stages, but not music. So as far as the battles themselves go, the 3DS has more.
(Unconfirmed) Game features like Event Matches, Multi-Man, Stage Builder, Replays, have a good chance of returning. And there are plenty of new game features: Smash Run, Customization, Wii U sync features, Amiibos, and they're actually used in core gameplay.
Sakurai even flat-out compared 3DS trophies outnumbering Brawl's trophies. There's only a few categories where Brawl still beats out the 3DS, and the game's not even out yet.