Traffic Club rotates your domains between the three parking providers DomainSponsor, Google AdSense for domains and Skenzo in order to find out where they perform best. So Traffic Park is actually not a parking company itself but a useful tool to optimize your domain names' earnings. In addition, they have a higher revenue share than what you would get if you parked your domains directly at one of the parking services listed above.
By the way, iMonetize.com is similar service supporting more parking programs... In case you want to give them a try, too.