Well, Nowi!Morgan gives Manakete twins with access to all normal classes and tactician, and has the bonus of being one of Nowi's less disturbing S-rank conversations (though it's still a little unsettling). On the other hand, Kellam!Nah!Morgan, with a +Def Avatar, A-rank Nah pair up and max defensive rally has, I believe, the highest defense in the game.
Tiki has much better speed than Nowi, though, and leaving Lady Forever Alone out in the cold isn't very nice. In fact, mag is Tiki's only negative stat mod, and the only thing Manakete Morgan would consider using it for is Ignis.
When casting aside Armsthrift, remember that Manaketes are primarily tanks and thus burn through Dragonstones very fast. While they're certainly cheaper than forged silver/braves, buying lots of Dragonstone+'s on hard or above without DLC can become prohibitively expensive.
One final note, Manakete Morgan uses Nowi's model. You may find it rather off-putting.
"it's that game that is so great it makes you want to destroy your Toilet." -Throwback2780 See how my mustache moves in the wind!