The fact you're having to ask probably means automatic is best
More seriously the only reason to use manual is if you have specific DNS servers that you want to use.
Some people prefer to use other DNS servers (Google is one option) because perhaps surprisingly they sometimes respond faster than those of their own ISP. In practice most of the time it probably makes next to no difference because DNS queries are not made all that often and modern OSes cache the results anyway (though the duration of the cache depends on the domain). Websites with lots of external content (eg;advert heavy) can sometimes load faster with a more responsive DNS server but I question whether anyone really notices most of the time.
A better reason to change DNS is if your ISP's servers are being used to implement some kind of content filtering (eg;site blacklisting).
But if you don't have any reason to change stick with automatic. It keeps things simple