M!Kana vs Nowi.

Going from stats, I think M!Kana has the better enemy phase. Going off of neutral stats, Kana has 6 more speed, and with the addition of Null Follow Up, speed can be fairly important for dragons and armored units. Nowi has more attack, but Kana has more defense and resistance (but Nowi can surpass one of his defenses by taking a boon in either stats).

I would go for Nowi. Nowi is easier to get copies of and does not need to rely on aether raids to get Holly Grails and such. The two units are very similar, and essentially have the same builds. Nowi has higher base and hp and attack compared to Kana, at the cost of 1 point of def and 2 points of res, and only 1 point lower in the BST bin compared to Kana. Personally, I would go with Nowi due to the higher damage output and stellar HP. The higher the HP the better because you need to stay in QR Range.

At full investment, I prefer Kana easily. 49 HP is all you need unless you want to panic ploy (so just get a teammate to do it instead), and he's much faster and even has better defenses at the cost of some atk.

However, merging GHB units is extremely expensive in grails. You might need over 6-8 months to get him to +10, and you would also have to give up the closest alternative to fortress def/res. Meanwhile, there's a lot of fodder like infantry pulse that you would need to delay redeeming.