Which unit should promote to 5S?

Catria and Lukas have perfect IV's, it just depends on what you're lacking right now. Catria is a great duelist and her kit emphasis's her niche as a duelist, +atk helps her attacks pack more of a punch while res is basically neglected for her.

Same for Lukas but he's slower and more defensive, +atk basically patches the -3 atk from Fortress Defense 3.

Personally, i would go with Luka. He is basically immune to red lords. And giving him bonfire with vantage will make him super annoying to deal with unless attacked by a mage. Catria is a well rounded unit, but i would only pick her if you are building a flier emblem. Pick nowi if you lack magic attack unit, yours have decent IV too.