Can I ask how far into the game you are?
If you can only raise one atm, and are still working your way through the first seven sigularities you should focus on Ushi.
Ushi is a HARD HITTING single target rider, and those are probably the single most valuable type of servant for clearing the story. Many of the bosses you will face will be ???* class and they are strong against knight classes (Saber/Archer/Lancer) and weak to calvary classes (Caster/Rider/Assassin) and beserker classes.
Further, all Casters and Assassins have a 0.9 attack modifier (meaning they can only hit for 90% of their attack power) and most of them are weak hitting support servants. So you, if your roster is weak you will want a hard hitting Single target rider.
Now, if your roster is pretty strong and/or you are done with part 1 of the story then you would most likely benefit from raising Astolfo instead.
By this point you are probably farming more than you are worrying about finishing the story and Astolfo has a an AoE NP and a 50% battery…the perfect farmer. He will help you farm QP during the 40 AP doors in particular.
Seeing as your servants are all 1/1/1 and you claim to be new I am willing to bet that Ushi will be the best investment for you currently. But, when you do get mats to spare throw some statues Astolfo’s way.
They are both good servants worth raising.
The first few ??? class servants are BEAST I servants. They are a part of Goetia/Beast I the final boss after you beat the seven singularities