Yeah the pluses/minuses I see for each are;
Godly Armor is usable by any Norse person who can obtain it, which is a potential boon because it prevents things like Odr based discrimination, and can be a boon even for young norsemen. It's effects are likely more set, which is a plus and a minus; you know what you're getting when you put on Odin Plate. But it means it's not always going to fit what we want. Is Hallbjorn going to be an Odin plate type person, or would he prefer we venerate, idk, Baldr? It means it's not one size fits all.
It also relies on outside forces; what if the gods get tired of lending their might or oppose the action being taken with the shared might? We don't really know. Tying back to young norsemen using it: extended use of its benefits is likely to stunt Orthstirr growth. Probably not hugely at first but over time as more people start relying on it more often.
Fylgja Armor only requires an unveiled Fylgja theoretically, but probably also requires it be powerful enough to Actualize, which seems to be a difficult process that we don't know when it's even available. It would be theoretically endlessly variable like Knightly Armor, with Halla being able to hand off her owl armor to Sigurdr and he has Albatross Armor, making it easier to pass down. But that also means we don't know what it will do until we first use it - if it's not particularly relevant, we can't really change it, but we can probably trust it would be relevant-ish.
However, it's use locks up some other uses of Fylgja stuff, such as our infinite shield spam strategy we use, or actualizing it in other ways such as a weapon or other combat aid. No major cost otherwise though, doesn't expend any resources besides needing to craft it.
Spirit Armor as posited most recently by me has the downside of requiring Odr, which instantly makes it even more elitist than being able to afford to make full plate already is. Depending on the spirits contracted the powers would vary much like the powers of a godly armor, but with even more adaptability than a Fylgja armor most likely - because theoretically we could change what spirit is inside it, by ending our contract with one and finding another. This would make it the most variable to situation armor, but requires Odr, being able to talk to spirits, and being able to negotiate with them to do what we want. All easy tasks for Halla potentially, but maybe not for someone else. I also expect the power to not be as high as Godly Armor at its maximum output, but that's my personal read.
It's also completely unproven to even Be possible, unlike the other two.