As noted, they have different purposes. But I think we really need to look at BOTH. Synastry often explains why two people are drawn together, and how THEY experience the OTHER person. It's how one chart impacts the other. Ever known a case where one person was just SMITTEN by another, who had little or no interest? Look at synastry.
In synastry, when you see conjunctions and mutual aspects, pay attention. For instance if BOTH charts have Sun-Venus sextiles, then there will likely be a strong *mutual* affection. (Sun-Venus isn't necessarily romantic, but it can be, and it's really helpful in both romantic and friendship relationships.)
Synastry can also show important ties like mutual reception of cardinal points. It's not uncommon for couples where one person's Asc is the other's Desc sign, or for it to be Asc/Desc and MC/IC.
As noted, composite is about *the relationship* itself, how it develops as an "entity" of its own, as well as how others "outside" the couple experiences the couple *together*.