What's more interesting is always subjective. And whats more beneficial to fundamentals is learning and enjoying the game you like (more). There isn't a set game that will just instantly give you godlike fundamentals. Just play the one that looks cooler to you at first, and if it's both, I guess just play both.
Hurtbox extensions can happen after active frames, which is why I mentioned may actually whiffing 5H here. We only upload active frame hitboxes on dustloop generally.