underscores or hyphens which is better in URLs

you might have seen a lot of sites with dashes and underscores, for example

1. some-other-site.com has used dashes

2. some_other_sites.com has used underscores.

The question is which one is better,

Google considers dashes or hyphens as separators on a site and underscores doesn’t effect (we can not say if Google changes its policy later)

so I will take above example,

in example 1 it is some other site (as keyword or search result)

and in 2 it is someothersite.

i hope now you understand which is better for you.