Halliday (2008: 161):
Grammatical intricacy [logical metafunction] and lexical density [experiential metafunction] are two ways of managing complexity: different strategies for transforming complex phenomena into edifices of meaning [ideational metafunction]. They are not intrinsically tied to the speech / writing complementarity; they derive from two variants both of which evolved in spoken language as a typological variable in the construal of complex processes.