Between these two options DuckDuckGo Browser is at least free and open source, while Vivaldi is closed source, which makes DuckDuckGo Browser the better choice.
Firefox and its forks are better than both. Firefox’s Gecko engine is independent of Google and Apple, while Vivaldi uses Google’s Blink engine and DuckDuckGo Browser uses either Blink or Apple’s WebKit engine depending on platform.
uBlock Origin on Firefox with the annoyances filters enabled takes care of that for me.