Actually, as far as barrier goes, onoki is better than yamato. Since Onoki buffs all 4 while Yamato only himself and the Earth main. WB doesn't do a heck lot of damage himself so that's not such a big deal, but iruka is a major source of damage.

4 buffs x 3 turns = 12 turn-buffs where as 2 buffs x 4 turns is only 8 turn-buff.

The thing that makes yamato better is the fact that he deals extra damage to fox. It barely sets him above onoki, if at all.

I mean Yamato does have before-attack-special while barrier is up, but onoki also give extra attacks. And Yamato only gets to use normal attacks 4 times (thanks to lightened border, he gets a total of 6 action in the 4 turns, 2 of which being barrier up, so 4 attacks, probably 0(1)-2-1-1(1)) which more or less cancel with onoki's unreliable normal attack. Granted, onoki's self damage will make him lose WB buff, but he have a mystery attack to compensate for that.

So in the end, the thing that sets them apart is the fact that Yamato deals extra damage to giant creatures, everything else matches up. And tbh, it's a bit of toss up given onoki's barrier advantage.

The one thing that can "seal the deal" is the fact that you can get Yamato to 5* easily, so he will do even more damage. At the same star level, I'm honestly not sure which would be better.