Everyone has p.much covered things but specifically on the transitioning MRD/WAR is better to go from into DRK primarly because of how their skill combo's work
both WAR and DRK start all combo's from 1 move which then branches into 2 options, one purely for agro/threat, the other for more utility or dps based direction
the dps direction then splits AGAIN into 2 paths, 1 is utility, the other is healing
While the exact effects obviously dont match, the core outline and framework does.
Butchers Block combo = Power Slash combo
Storms Path combo = Souleater combo
Storms Eye combo = Delirium combo
You even learn them at the same levels.