It's quite interesting really. Both the Didact and Vader have a universal force in the palm of their hands. That's a tough one actually. Vader, while being incredibly powerful in the abilities of the force that he has access to, along with being cybernetically enhanced, is a hell of a powerhouse, not to mention his abilities with a saber. While on the other hand, the Didact was the military commander of all the Forerunners, and the best tactician they ever had. His suit is essentially MJLONIR but many times better. He's got control of gravity with the flick of a wrist. Gravity itself is just about as flexible as the force when it comes to possibilities. Personally, I think Vader and Didact are going to be having a lot of choke hold struggles over each other.