For me, I like to look at the synastry to see if there is any hard aspect between Saturn and the personal planets, because I read that these are very binding. I then look at whether important planets in one chart activate the angular houses in the other.
One thing I want to point out is that you might also want to look at transits. Natal charts, synastry charts, the composite are all static, however humans aren't static, they are always changing, and not to mention circumstances change all the time too. So, maybe normally one of them is not the type to think about marriage but perhaps in the transit an important planet is activating his/her descendant, or something, and that can tell us things that the more static charts can't.