Single Signature Wallet;
12 word seeds for brain wallet.
24 word seeds for storage in perpetuity.
Multi-Sig Wallet;
12 word seeds for convenience.
24 word seeds for stealth (useful as a dummy single sig wallet).
Discussion
I prefer 24 word seeds if you're creating your own entropy, through randomly picking bip39 words out of a big bowl.
The preference for 24 words comes from the extra randomness achieved through mixing & blindly choosing 24 times vs 12.
If you don't have patience to cut up all the words yourself support #[2] he sells the entire word set 3d printed!
Makes a nice memento / bitcoin conversation piece too. 🧡 https://btc-hardware-solutions.square.site/ 
This is the most powerful set of plastic pieces on the planet.