Cordelia is overall better as an offensive unit probably, with her statline making her offense ridiculously forceful, but Valter seems much more flexible. He may not be as offensive, but he should still be dangerous enough, especially with quicker specials. His good Def, decent Spd and fast specials means he can function as an enemy phase unit as well, which Cordelia cannot do, and, to my understanding, is something Flier Emblem does not have much of aside from Green.