I am a surveyor/lawyer (don't ask!). I use M (latest version) for lengthy reports, I find it excellent, particularly autostyle, footnotes and line numbering.
I use Pages '09 for writing letters, usually one or two pages, rarely longer than half a dozen. Also I have Nisus Pro that I sometimes use for opening file formats.
I save all outgoing letters, etc to pdf, and keep the M or P letter to use as the basis for the next letter. Email incoming communications are often .doc or .docx (sent by Word users). To open .doc, I use either Nisus Pro or Pages, then print to pdf. (I suppose I could use M to open .doc/dox but hadn't thought of doing that before.
I am changing my current imac (running OS 10.6.8 Snow Leopard) to a new IMac with OS Sierra. Sierra's version of Pages differs from the '09 version and from previous experience i expect to have to update templates and all existing saved 09 files to the Sierra version. Rather than continue to use two word-processors, I am considering confining to Mellel.
I'd appreciate please some input as to the advantages and disadvantages. Thank you.