I agree that the 2 are very similar – both strive to simplify the knowledge of how the human body works.
FMS just uses a slightly more structured approach where you can actually use metrics to track progress.
I don’t know a whole lot about FMS – but my guess is that a blended approach between the 2 would be optimal for people with medium deficiencies that are wanting to focus on form and full ROM in a crossfit environment. Kelly’s is probably best for athletes that are already most of the way there. FMS could really benefit those couch potatoes that are just getting off their asses with TONS of mobility, muscular imbalance, and functional issues.