I tend to use Synastry much more often than the Composite.Synastry tells me the foundation of these two people meeting for the first time and their general interactions. Whether it will be sexual, affectionate, intellectual, or whatever.
After observing the synastry in action I use it to guide my composite because with the synastry you'll know what the relationship is based on but the composite will tell you how it comes together once the two become a unit.
I would say I never get past synastry reading with people I meet 80% of the time because honestly, with most people you meet you won't need a composite.
One of my clearest examples come to mind where the synastry was loaded with sexual attraction through Mars-Mars,Mars-Asc,Venus-Venus and all sorts of other contacts like Sun trines, and Moon stuff. But very poor mercury contact.
The initial meeting with this person was great. We liked each other instantly on all levels, and the heavy Uranus activity made it energetic and fun. But our relationship really solidified a few months later as we bonded over Sunday happy hour discussing my usage of astrology, and her uses of the I ching and the enneagram and all sorts of human psychology stuff/spiritual stuff.
We taught each other a lot. But the answer wasn't in our synastry Mercury contacts. Which weren't too bad but not good. Well at this point we were many months into knowing each other and in my opinion more in the realm of composite. And the composite Mercury held all the answers.
Composite Mercury in the 3rd House created a constant exchange of ideas.Two trines from Neptune and Mars, and two Sextiles from ASC and Pluto. Pluto being the most active. Our 3rd house Mercury was dominated by the themes of Pluto and I have never learned so much about myself, others, and spiritual topics with anyone else. It also gave the mechanics for us to talk sense to each other before one made a mistake.
So! Synastry is great for the initial clash of planets, but eventually, at some point, in some arena, the two people may become partners and the composite aspects will play an increased role. But generally, unless the relationship begins to become particularly important, I save my composite reading as synastry is usually sufficient for the Hi and Goodbyes in life.
I think I'm better at interpersonal astrology than natal or transit cause my 7th house Sun is always thinking about others