Marshall is the father whos in his 40s to 50s and Forrest is the son who is Caucasian Chinese so he closely resembles his father in a more American type way
Forest being younger I feel has more promise than Marshall in regards to story potential. He could've had a rivalry with Hwo and led the next gen with Hwo, Jin, & Forest. Could've made for some cool story context. Considering how Marshall became a joke from Tekken 5 on, it can be seen that they don't know what to do with Marshall...
and so they try to put him with Paul & Steve which that was awkward. And Steve to me fits better alone with the story he has now. And now hopping into T7 and Law's still a joke story-wise. With Forest, you could've gone many different routes. He was literally that cool of a character. In fact Forest was my first main. And then added Jin and finally went to Hwo.