No significant difference. minor: To me, posted suggests that both people know something that may happen, as if they might want to know WHEN it happens. Informed is more general, so can be usd in the same place to mean something known, or very generally about anything that might happen. . But, mostly they are just used for the same thing.

''I'll keep you posted'' infers that new news is likely to be announced/said/addressed/pop-up in the future, and they'll let that person know about it. ''I'll keep you informed'' implies that if anything does come up in the future, then they'll let that person know about it.