I’ve noticed that most writers tend to either stick with triads OR use upper extensions (7ths, 9ths, etc.) a lot.  Often those ‘choices’ are just habits.  Making such dramatic ‘all or nothing’ stylistic choices may not be in a particular song’s best interest.

In some styles of music it’s fine.  If you’re playing traditional folk music, you’re going to mostly use the simple beauty of triads.  It’s often the way to go in many styles of music if you want the harmony to sound more ‘straight’, less inflected and full.  But for some songs it may sound too colorless.

Conversely, regularly using upper extensions of chords (7ths, 9ths, etc.) in a song will give the music a more lush, often jazzier, feel.  But when used indiscriminately it can take character away from the chord changes, having a ‘kitchen sink’ effect, harmonically speaking.

I’ll make the case, though, for not getting stuck in one or the other.  Often a couple of 7th chords dropped judiciously into a sea of triads, or some triads in a song with a lot of 7th chords, can be incredibly effective.

Most of the time it comes down to asking myself – which sound best supports the melody and the lyric?  (NOT ‘which sound is in my current comfort zone?’)

Let’s say you know your chord in a certain place will be an Aminor.  It won’t hurt to experiment with an Aminor7, Aminor9, etc… just to hear what it sounds like with the melody.  It will not necessarily be out of character for the song.  In fact it can help create a stronger character for the harmony of song.  But sometimes nothing will be more effective than leaving that straight Aminor triad alone.

Or let’s say you have an Aminor7.  What would it sound like as a simple Aminor, without the 7th?

You want to keep the song ‘in character’, with a consistent tone, but you don’t want your harmonic sound to be entirely predictable either.  ‘In character’ does not have to mean predictable or monotonous.

If you’re someone who uses full harmonies (often piano players), try opening your ears to the austerity of triads.  It might sound flat and dull at first, but give it a chance.

If you’re a triad person (often guitar players), try opening your ears to some other sounds.  Start with 7th chords – often underused these days, and therefore kind of fresh-sounding.  It may sound weird to you at first, but don’t give up.

Think of this like wearing clothes of a color that you’re not accustomed to seeing on yourself.  Sometimes it just takes a little getting used to.