Knives vs. Staves: Soren's Personality
It's a no brainer that giving Soren staves is better for game utility, but when people want to base the choice off of his personality, they often go for knives.
I'm going to argue that choosing staves actually fits Soren's personality better in multiple parts:
What would go into Soren's choice of tool would be his motivations and priorities.
The way I see it, Soren's #1 priority in terms of motivation is: "Will this course of action help Ike survive?" And here we can see two outcomes:
Outcome A: Soren gets staves. In addition to the skilled combat he is still capable of due to his magic, he now has the ability to heal Ike if he gets hurt.
Outcome B: If Ike gets hurt, Soren can't do anything. But on the bright side, he can choose to get himself hurt instead. Or just continue to do magic, pretending he never got the knives.
Outcome A significantly improves the success of priority #1.
Now onto priority #2. Assuming Ike will not be in any danger, Soren's next motivation would be "what is the smartest course of action, regardless of people's feelings on the matter?"
Now the interesting thing about priority #2 is that by its very nature it matches with the better option for gameplay. Why are staves better for gameplay?
Soren is too weak to do any notable damage with knives
Soren puts himself in more immediate danger while trying to
As with #1, Soren can't do anything if Ike is hurt (or anyone else)
Soren would no doubt consider all of these factors himself, making staves better for priority #2 as well.
Ok, so assuming both of those priorities are fulfilled, Soren will now move onto priority #3: "what can limit my contact with untrustworthy people?"
Well, if Soren uses knives, he will have to face enemies 100% of the time. If Soren has staves, he can spend a good portion of his time with allies, or even just the core Greil Mercenaries, or even just Ike if it suits him. So staves win for priority #3.
Now, deep down, we might run into priority #4: "is this useful for getting back at those who hurt me?" But something I've noticed is that there's a tendency in the fandom to make Soren more vengeful than he is, similar to how they make Ike dumber or more obsessed with training. This priority is so low on the list, I'm not even certain it really exists.
I'll get back to that in a bit, but for now, let's assume priority #4 is in place. Knives will definitely be better than staves here, but with a massive caveat. Who hurt Soren the most? Dead people, civilians, laguz, enemies with absurdly high defenses and physical offensive power. Dead people are obviously not someone he will face, nor civilians. There are very logical reasons to use magic against laguz instead of knives even if his strength wasn't negligible, namely their race-related elemental weaknesses. And as for enemies with absurdly high defenses and physical offensive power? Soren shouldn't fight them at all if they can counterattack, and definitely not with a knife. More pertinently, Soren doesn't even recognize them (and vice versa), making the vengeance motive less relevant.
But then the other question is, just how vengeful is Soren anyway? He doesn't like people, especially laguz. He shows this by avoiding them, acting standoffish around them, and if he's really upset, insulting them. But even if it sparks a physical fight (which happens once), he's never the one to escalate it to violence.
Take the infamous fight between Soren and Lethe and Mordecai. Yes, Soren's comments were uncalled for, though as far as Soren was concerned, so were Lethe's, which is what caused the conflict in the first place. But it was Lethe who escalated the fight to violence by commanding Mordecai to kill Soren. Which Ike was clearly not ok with, jumping in front of the attack to protect him. Then, and only then, does Soren resort to violence. If anything, Soren seems surprisingly un-vengeful given everything he went through.
But there is a character who fills this niche people are looking for with Soren, and his name is Reyson. Reyson would absolutely choose to wield a knife if he could (or even if he couldn't).