Fuuma Kotarou vs. Emiya Assassin?

Fuuma is the best budget crit star gen bar none right now. His abilities synergise well with what you want for your raikou. Targetable evade on demand that also boost crit drop rate and a debuff to indirectly increase her surivivability. Using kotarou because I had no Assassin(didnt really like Cursed Arm till Camelot) and got a NP4 copy of him from Onigashima. He does pathetic damage but makes up for it with almost Jack levels of crit star gen. So yeah ideal team comp is already waiting for you. Kotarou, Mom and waver/Hans/Mash. Assassin Emiya is more of a mixed bag rather than a specialist like kotarou

I dont really know emiya but i do have fuma and already at final ancension. I use him with raikou and i can say he is a good servant for star generator but outside that he is very weak,low damage, and low damage np. If i have choice i Will lvl up emiya since his skill and np more useable than fuma
Ary for bad english

The thing is, they both do quite different things. Fuuma is a quick-based Assassin who generates just as many stars as Jack does and he is more accessible than she is, so putting him on a team for crits is ideal, he also has a targetable evade so he would go quite well with Raikou.

Kiritsugu is arts-based and can spam his NP and generate quite a reasonable amount of stars. SO, I would recommend focusing on Fumma to go with your Raikou, but go back to Kiritsugu later.

I have both, with Fuuma being grailed to 80, and personally, Fuuma is better. As a crit gen, he will definitely help your team, being a poor man's Jack. His evade is amazing and can help your DPS survive NPs considering the ones you listed lack an evade or invincibility.
Kiritsugu's QQAAB works better than Kotarou's QQQAB with the team you ask, but his attack down and evade does help your team survive. But do check out Kiritsugu because that targetable taunt does have a great niche which can help.