Oni's got bigger arms.

Both are strong...or were...or will be. Oni used to be great but they've messed him up and are planning on fixing his flick out of demon dasb in the future, which is what made him fun. But he requires a bit of work from the player to do well.

Spirit is the opposite, not really demanding much from the player other than "do you have ears and if so, do they work". Her power's a guessing game for the survivors and you can basically guarantee a down after they're injured, making her really strong, both in terms of power and how much she infuriates survivors. Her lunge out of phase is also bugged too (unless they've fixed it now) but she doesn't rely on it to be effective.

In terms of perks Spirit's are better by a long shot as well.

Spirit mains are nerds don't fall for the trap

Oni isn't really my cup of tea, I've had good matches with him and I can't see to enjoy him as I do with Spirit. Using Spirit I get a thrill out of being outplayed, and coming back to correct that mistake

Depends on your perspective of what makes a killer desirable. Don you want a killer who performs well when played properly? Do you want a killer with cool aesthetics? Do you want a killer with fun lore or interesting power?

I can offer my opinion on the matter but as mentioned above you will need to decide what traits you're after.

-Spirit is a great killer if you want to stand a chance against top teams/players or simply wish to focus on ranking. She has a somewhat difficult learning curve if you are someone who typically plays by sight and not sound. While headphones are not needed to play Spirit they do wonders for her. Spirit has a fun power allowing you to use it for both map pressure and hard chase engagements. This killer will definitely make you feel like a powerful force rather than a fumbling plaything. Be warned you may see tons of DCing players.

-Oni is an interesting topic. I play PS4 and connot speak to PC where it seems he is viewed quite differently. I can say that on PS4 frames tank when you use his power "Demon Dash" (for both killer and survivors nearby). Sadly due to poor frame rate its a damn miracle if you ever land his ability as his "insta down" he can be dodged by simply moving left or right with more ease than a Billy chainsaw. While visually pleasing, this sexy hunk of demonic man doesn't measure up when it matters most. The character is still fun for the fact that you feel the weight of each step (much like Demo-goodboy) and you will feel like a truck built of muscle and testosterone, although this feeling will be fleeting vs a competent team who understands basic dodge/loop mechanics.