That is interesting. But i don't understand yet why there is such a big difference in safety for having that passkey as a password for the wallet.dat or having it as the seed for a private key. Where does the difference come from? I mean bruteforcing should work at the same speed for both isn't it? Or are there iterations of the pass for the wallet.dat so that the time to bruteforce gets extended?
There are two functional differences:
1) For wallet.dat encryption, they need your wallet file, and can't attack your account without it.
2) Even if they have the wallet file, they have to expend their effort attacking your file. In stark contrast, attacks against brainwallets attack ALL brainwallets simultaneously.