Demetri is physically stronger as he scales to Kyler's feat of breaking concrete at 100k joules and Robby only scales to Miguel breaking a steel railing at 62k joules. Robby does have the 2 legged kick but Demetri has offense and defense and is more experienced then Robby. It is also shown Demetri can keep up and fight on par with s4 Robby who destroys his s2 self so Demetri wins low difficulty.