Love DaniJade's response to you. I've spent a lot of time looking at synastry and composite charts over the years. The problem with it is that it's looking at specific aspects and none of the parts truly considers the whole person. You can have a very positive-looking synastry or composite and still not have a good relationship.
As an astrologer, I may look at synastry charts and do a composite chart, but I will lean on comparing natal charts the most - to truly work with the whole person. Still, astrology can never know for sure who will work out the difficult parts of any relationship. It can point out what could be difficult and what will require effort to work out - which is of value - but truly, I think it's best to use your intuition and follow your heart. Even relationships that don't work out in the long haul can be very meaningful - and provide a person with valuable experience.