I was planning to do this smh. Prepare to hear from my lawyers
Salem's profile doesn't have crazy good scans, so I'ma do the best with what is there and what I remember.
Both can one-shot each other, but Salem has better regeneration since hers isn't limited to the 126.1 million lives in Japan. So in theory, if it just came down to who can outlast, Salem should have an advantage. Both do have ways around the others' regen to in, however, with Makima having a lot more options to do so. Makima can BFR with the Hell Devil or the Eternity Devil, madness manip her with Cosmo, or turn her into a doll with Santa Claus. While Salem also has a few, destroying Makima's soul, Apathy Grimm, and
maybe freezing Makima completely with ice, Makima overall has a lot more ways to counter regen.
Salem should be able to avoid Hell Devil BFR by turning intangible, but she doesn't have a good way of escaping the Eternity Devil's pocket dimension. She might be able to cause it pain til it offers her its heart like Denji did, or use her SI to talk the devil into giving up, but since Makima's controlling the Eternity Devil here, SI might not work. Dollification might not work on Salem or the Grimm due to them
not being human, but if it does, Salem can prob avoid it by becoming smoke. With all that being said, Makima's best wincon is Cosmo since Salem doesn't resist her ability, and it's quick and easy to use. Salem's best bet around it is prob having Apathy Grimm immobilize her before she can use her ability or by having a Grimm one-shot her, but with how fast Cosmo can use All-Out Halloween, it's gonna be hard to do so.
Makima doesn't resist having her soul absorbed by Salem or the Grimm. Considering Salem can control 1000's of Grimm and
Makima's habit of not avoiding attacks, she might have her soul destroyed by one of the stronger, random Grimm attacking her (It's also worth noting that Makima prob can't control Grimm since Salem is already taking full control of them). Makima also doesn't resist Emp Manip from Apathy Grimm, which can quickly leave her and whoever she
summons completely drained of will and immobilized. Complete freezing prob won't work since it is an attack and thus
can be transferred to some citizen.
Overall, I think I'm leaning towards Salem since she has slightly better ways around Makima's wincons. While Makima doesn't have many ways around Salem's main wincons of herself or a Grimm absorbing Makima's soul or incaping Makima with Apathy Grimm.