You just cant compare the two...afterall Sephy could easily survive the Shun Goku Satsu mainly because as a god he could handle a few piffling demons trying to beat him up...and you may say that cloud manged to beat him so why cant Akuma well simple answer is: Materia. Akuma doesnt have or want Materia but Cloud and co did.

Another thing is Bison's "Phycho power" (terrible name for it by the way) is no where in the realm of even normal materia users powers as its just essentially just telekinesis and a bit of energy throwing...so you can hardly make out that by defeating Bison that Akuma is the best warrior out there.

This whole arguement is rediculous mainly because your comparing two COMPLETELY different people from a completely different Genre and completely different world.

Anyway all Sephy would have to do is use the black materia and Akuma would be history anyway. Afterall i dont see him being able to get off the planet.