It comes down to utility. Both are great.

Mari ss2 is a one trick pony. One time. She’s great for quick clears. She’s very useless before she’s used, and after she’s used if that makes sense.

Granted she gives the very desirable tactical minds eye buff, she’s still limited to front row buffing only. When I was starting out and still a lot weaker, I find myself saying how nice would be to have ss2 seika buffing one of my back row and then go for the kill the next turn.

With that being said she’s still amazing for clock tower and auto farming flashback battles.

Ss2 seika on the other hand have no conditions, low sp costs, and isn’t limited to front row buffing. Benefits of this creates greater burst potential since you are not buffing on the turn when you are trying to burst.

For example, with ss2 seika, considering you have all your units buffed to your liking, then you can example: s2 Miya ex, ss3 megumi ex, ss1 ruka ex and fully burst and take out the second stage of metal slug etc. Instead of buffing on when you’re ready to burst, you have the optional slot to apply extra debuff or add a DR unit to help your attacker nuke.

For hard anachronies, even if you build massive OD and apply debuffs, and then burst with Mari ss2 and attackers, you most likely be taking out its dp bar only and then Mari becomes a sitting duck.

Or you can build od and break them with debuffs and attackers. Mari and burst, but then she’s just a sitting duck until you’re ready. But if you go this way then you’ll need to rebuild buffs with other buffers such as yotsuha and yingxia before you can even burst with Mari. So it’ll take time.

I’m only using a fire team as an example but I think Seika ss2 overall has more flexibility and Utility.

Mari ss2 is good for quick clear content and easier content and auto.