404 error save ?

Hi,

Thanks for contacting us here about this extension.

1// So is it possible to save error 404 somewhere with this component ?

You can redirect the 404 URLs. For more information:
https://www.joomunited.com/documentation/wp-meta-seo-documentation#toc-4-1-index-404-error-urls
https://www.joomunited.com/wordpress-products/wp-meta-seo/404-errors-and-url-redirect-manager

2// And is it better or equal compare to yoast ?

It depends on many factors. However, we have some features that Yoast SEO does not. You can see our comparison here:
https://www.joomunited.com/wordpress-products/wp-meta-seo/wp-meta-seo-vs-yoast-seo-comparison

Hope it helps!
Cheers,