Gecko is the browser engine developed by Mozilla and used by Firefox.

It's better for adblockers, since Google blocked access to features used by them in chromium.
For a performance comparison check this out.
From a security point of view, Blink/Chromium got more people working on it (Google, Microsoft, community, etc.) so potential vulnerabilities get fixed faster but by being way more popular, way more vulnerabilities are found regularly and exploited in the wild.