Public or ISP DNS Server?

Which one should you use?
Public DNS Server like Google Public DNS or OpenDNS can be used for faster browsing and other benefits. But in some cases, your ISP DNS Server might be faster.

Recently, Digital Inspiration shows how and why this happens.

It's all about your IP address. As Digital Inspiration explains, all of the major websites use Content Delivery Networks like Amazon and Akamai to serve up the content. A CDN looks up your computer's IP address to direct you to the nearest server, but because a public DNS server passes requests anonymously or without your precise location, the CDN might serve you content from a server that's actually not the closest one to you. Your download speeds will thus be slower than if you were using your ISP's DNS server.

Google's DNS server information page acknowledges this issue:
Note, however, that because nameservers geolocate according to the resolver's IP address rather than the user's, Google Public DNS has the same limitations as other open DNS services: that is, the server to which a user is referred might be farther away than one to which a local DNS provider would have referred. This could cause a slower browsing experience for certain sites

How to tell if you should use your ISP's DNS server or a public one? Digital Inspiration recommends (and provides instructions for) using the dig tool and an IP geo-location app to find out if the CDN servers you're accessing are near you ... or on a different continent. You can also use previously mentioned Namebench to find the fastest DNS server for you.

On the other hand, resolving DNS is unlikely to be an issue for most people. The difference in speed between Public and ISP DNS Server are not that noticeable that you'll shout "OMG THIS IS VERY VERY SLOW!". But, with Public DNS Server, you can bypass some blocks that your ISP has been enforcing.