Brave is a standalone browser; Swash is compatible with Brave and all major browsers. As an app, Swash has more flexibility and greater reach due to its cross-browser and multi-device perspective.
Brave/BAT is focused on selling advertising space and creating a new ad ecosystem. Swash offers this with Swash Ads, but alongside a wide ecosystem of other data-related products. Data is also something that can be sold multiple times, bringing more value back to Swash users.