If I had to choose 1, I'd choose the nsec.
The 24 words could be an optional way to back up & restore your identity. It shouldn't be the only way though.
The copy & paste functionality is the main reason I'd go with the nsec. This isn't money & currently is just the keys to a social identity.
In time the nsec might become more critical to protect & offline storage of this secret might make more sense.