The speed difference between the two during real world browsing has been negligible. In fact, a recent BrowserMob article has me reconsidering the plain-old Comcast DNS servers.
Download the Java utility from BrowserMob to run your own speed test.
java -jar browsermob-dns-perf.jar
To test your ISP’s DNS as well, add the IP address(s) to the command like so:
java -jar browsermob-dns-perf.jar 123.456.789.012