Bip39-Dice VS monero-wallet-generator

BIP-39 Dice

You can't really. That's why using open source software is so important. Or do things fully offline by hand to be sure: https://github.com/veebch/Bip39-Dice

You can make a wallet by rolling dice actually (or any other non-digital source of randomness). Here are some instructions: https://github.com/veebch/Bip39-Dice

https://github.com/llvllch/Bip39-Dice to generate the mnemonic, https://iancoleman.io/bip39/ to generate the address

There is, https://github.com/moneromooo-monero/monero-wallet-generator

You can use the offline version here by downloading it the .html file and opening it in your browser: https://github.com/moneromooo-monero/monero-wallet-generator

monero seed is 25 words already as you can see by generating a paper wallet over: https://moneroaddress.org which preferably would be using it offline by opening the .html file locally after downloading from: https://github.com/moneromooo-monero/monero-wallet-generator

Get the Monero offline wallet generator (remember to verify the signature) and put eit a trusted usb. Note: this code is written by trusted members of the comiunity and in very nice JS, and is very easy to audit for those curious things like that)

Download the Paper Wallet Generator.

Which method is better for a cold wallet creation - using cli-wallet or this tool?

go to https://moneroaddress.org/ in an incognito browser (depending on your security profile, you may want to install a system specifically for this purpose - maybe on a raspberry pi - and then shred it afterwards without connecting to the internet again; or download the html directly from the source and open it in an offline browser / dedicated offline system)