Neither of those ideas are any good really:

-   Committing to memory 12 or 24 words in a given order is not an easy feat, and just try letting some time go by to see how that plays out.

-   Having the seed on any online device can be subject to things like losing the phone, being hacked (i.e. malware), confiding in a random note app which may be doing God knows what and so forth.