Halliday & Matthiessen (2014: 655, 657):
These two resources complement one another in the grammatical realisation of transitions in text. The general principle of complementarity is this: clause complexing does relatively more work locally, while conjunction does relatively more work non-locally and even globally. Clause complexing ‘choreographs’ the local development of text by means of univariate structure, indicating both taxis and type of logico-semantic relation. Conjunction can work together with clause complexing, reinforcing local relations, but it tends to take over from clause complexing as the relations become less local and more global. Looked at from the point of view of lexicogrammar, this means that the local organisation tends to be ‘tighter’ whereas the more global organisation tends to be ‘looser’. At the same time, looked at from the point of view of context, the more global organisation is subject to more contextual guidance in the form of generic structure. In this way, grammar and context complement one another in their contributions to the semantic organisation of text. …
However, this pattern of complementarity varies significantly across registers. In particular, in unselfconscious spoken text, clause complexing does relatively more work and conjunction relatively less. Here clause complexes can extend well beyond local relations, reaching a span of up to 20 to 30 clauses. Conversely, in planned written text, clause complexing does relatively less work and conjunction relatively more.