There are two camps on which is better. The WTB rims are UST compliant, the Stan's are not. A UST tire will definitely work on a UST rim. Non UST talk mixed bag. Stan's might do better with non UST tires, might not. At one time there were not that many UST tires on the market. That has and continues to change as most companies are offering a UST version on their more popular tires.
Personally I would go with a UST rim and a UST tire because there is a standard that has been created and designed for the use of tubeless bike tires. That's not to say there aren't tons of people running non UST tires tubeless on Stan's, but the tires weren't designed to be.