I've had some killer grass fed from Kansas. It was finished with a sweet grass. My cousin finishes with corn, and his parents used some sorghum mixed in before him but I think he finds that to be fake magic.
The way I see it, corn fed more fat, larger in general. I think you have to dry age it get the 'beefier' flavor OP is referencing from the YouTube video.
I've eaten steak everywhere except but on the west coast. It's good everywhere, but I will say when I came back to Iowa from DC, I almost fainted at how much better the beef was.
I think they're both good personally, but it's hard to beat my cousin's corn finished Iowa beef. Best hamburger I've ever eaten.
We buy local grass fed where I live in Kansas and it it close to the best I have ever had. Not the best, but close.
The ground grass fed that we buy is actually the best burger meat I have ever had and it isn't even close. Everyone who I have cooked a burger for can't believe that it is only salt, pepper and walnut oil.