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I think you’re thinking of this in the wrong way. It seems to me that you’re thinking of the seed and passphrase of 2 different tools that both do the same thing, providing entropy.
I think you’re better of as thinking about it as 2 different tools that are optimized to protect you against different kind of attacks.
The seed is your source of entropy and is your protection against brute force attacks. The seed should provide you with all the entropy you need, and you should never expose your seed to the internet in any way.
As you’re unlikely to remeber your seed, you should probably write it down and store it securely somewhere. That however creates a new attack vector, a physical one. A passphrase is an additional layer of security that protects you in case the physical copy of your seed gets compromised. That way your bitcoins are protected by 2 things, something you have, and something you know.