One of my professors (in medical school) pointed out that these posters are great for learning the chemistry, but aren't very useful to us students because they don't show you what cells/tissues these processes are happening in.
There's also a lot that's not on either map, like specialized secondary metabolite synthesis pathways (although kegg has select secondary pathways).
I agree with you regarding their mutual incompleteness. There's plenty more literature to curate! See my systematic omparison of KEGG and MetaCyc for more details: