I would use Firefox over either, but at least Vivaldi's ownership model seems pretty good (employee owned). Brave is just so untrustworthy at a company level.
Better privacy, better UI not cluttered by excessive and unnecessary options, and a fantastic feature set... but I'm referring to their search engine in this context.
Because it objectively IS. It passes privacy tests in so many areas where others fail and I'm not just talking about those privacy test sites, I'm talking tests we can run ourselves. It's fine if you don't like Brave but don't let your emotions get in the way of the fact that it is, by default, the best option for privacy out of the box. Sure, you CAN harden Firefox or use Librewolf but even those don't always succeed in passing those tests like Brave does.
The AI is intrinsically linked to the search engine if I'm not mistaken. Brave has AI built into their search engine and their browser, but it's very well done and pro-privacy.