The GT DD Pro is far cheaper, scores higher on our rubric, and is the only PlayStation-licensed base - a strong all-round entry. The T818 costs nearly three times as much for 2 Nm more torque, and its case rests on the Thrustmaster SF1000 rim ecosystem rather than the base itself.
So unless you specifically want the SF1000 wheels, or already own Thrustmaster gear, the GT DD Pro is the better buy on value, score and console support. The T818 is a niche pick for the ecosystem, not a value one.