There's no contest, .tk is better in nearly every way.
Both use DNS, both expire after a year, both have a premium option, they're remarkably similar. The deal breaker is that .tk is an actual domain name, while co.cc is nothing more than a subdomain. Another reason why .tk is superior is because it's a lot easier to set up because the settings are more straight forward and instant than with co.cc. With co.cc you have to wait like 10-20 minutes for your settings to update while with .tk it's usually instant.
The only beef I have on .tk is that they have a strict TOS. My .tk domain was terminated even though I didn't violate any rules, meanwhile the staff at co.cc look the other way to the domains that people register there.