Kotarou Fuuma or Hassan of the Cursed Arm?

Just star generation? Fuuma's a lot better specially when combined with his ninjutsu skill.

Fuuma has higher Star Generation and an extra hit on his Quick cards which in turn would generate even more stars. He provides a host of debuffs on top of his star generation. Hassan himself is only capable of providing stars and offers no other team support. Both are low on damage as well compared to other servants. Hassan however does have 2 skills that assist him in star generation which might make him more consistent in some cases, but overall I believe Fuuma to be the better choice.

Fuuma is more consistent with his star gen while Cursed Arm has more "burst" with his skills. Being Assassins both do pathetic damage and have low hp anyway. Difference and also the reason Fuuma is rated higher than Hassan is the targetable evade that is very rare to Fuuma. So pairing fuuma with a zerk like mom and herc can be pretty good due to his low star weight and debuffs that keep them alive longer.
You said it yourself Hassan dosent offer as much utility to the team. Something Kotarou has in Spades

If you have a support that can keep your units longer i say go with fuma. If not hassan because of his 3rd skill, and keep in mind that the purpose of these unit is to generate star which theu both do a good job at. I use hassan since he can survive longer with 2030 and he do more than decent job