Maid Alter or Umu Caster

Gameplay wise? I would recommend Nero Caster. Otherwise, pick the one you like the most.

In terms of gameplay, Umu is better. She is an excellent farmer despite being a caster: she has 50np battery and a ridiculous 3-turn %50 attack up that can be applied to others. Malter is a decent ST quick dps, but she will suffer competitions from many other strong ST riders, especially 4* Kintoki rider, who we will get for free this year. But in the end, it depends a lot on your current roster. And Raikou indeed is a very good lancer.

One is the best Caster AOE, with ability to Buster Brave Chain with her NP, NP battery up to 50% and some massive steroids.

Other is Rider ST Quick who hits like a truck thanks to a good 3 turns quick up + massive self-attack up to multiplicate, and will be god tier boss killer with Skadi, around 2020.

Choose by preference, if you want to decide by gameplay, look in your party and decide what you truly need. Has quality Rider ST Quick? Has good Buster AOE Caster? If yes for both, go for what you want.

ps: Lancer Raikou is a very good buster crit lancer, but if you have Lancer Alter Altria, she is kind of just her ST version.