Bulls with a castle are roughly equivalent in resources to Hydras in a fortress, so the question is, which is better?
As Comrade Forrest said: two Hydras on day 121 are powerful, but 6 Dwellers and others are not. I agree with him.

That's a very categorical statement. I don't disagree that 2 Hydras are good, but if the choice is between them and 6 Dwellers with a castle, I would choose the second option in most cases. The army will look more solid.

Hydras are not very versatile; they only show their strength if the monsters are fast and reach the first round (Flies, Dragons, Griffins to some extent). That is, if you have little money (only Hydras are enough to buy), but you see a lot of Hives, then Hydras are the only option. But if there is a prospect of breaking through numerous standard guards, then it is still better to have thicker main stacks.

In fact, you can compare them in terms of hit points and damage:
2 Hydras
vs
9 Bulls + 2 Dwellers + 2 Vassals + 8 Flies + 12 Lizards + 16 Gnolls (if I remember the numbers correctly).

Here, the Bulls alone already have an advantage in both parameters.

Plus, if you compare the strength of the army on the 3rd week (assuming the rest of the Dwellers are built on the 2nd week), the first option will give only +1 Hydra. And the second will give the entire list, minus 3 Bulls. That is, the difference is even more significant.

In the game, you rarely have to make the specified choice; more often, you need to think about whether to build Hydras or Bulls on the 7th day, given that everything else, including the castle, has already been built. But here, the choice is usually easy - it depends on the money. With Hydras in the castle, full purchases, and a decent hero, you can even go for a Consul-Maximal strategy.

But if you are short on money, then in most cases it is better to have Bulls or even more Gnolls - for faster purchases and gaining tempo, and leave the tough Cons for the 3rd week.