My experience using NAMEBENCH and other DNS testers is that the DNS server you select today as fastest may not be in the top ten the next time you test even if it is only moments latter.
The only benefit I ever have seen from selecting my choice of DNS resolvers is when I am using a VPN connection to a remote VPN server. I don't want to be using a DNS server in Florida when I am connecting to the Internet in London.
There could be some benefit in having your primary DNS and secondary DNS from different companies in case your primary provider's network and servers failed.