I imagine this would be a good aspect for a relationship, particularly for the longevity of a relationship, as Saturn is where we are supposed to look to for the long-term potential. Anytime Saturn is in positive aspect to a major planet, I imagine it is adding structure and support and discipline to the affairs the planet it's aspecting deals with. Trining the Sun- I see it as a relationship where the sense of self- and self-expression of the two individuals is being supported - you help each other find yourselves, perhaps? If it was in negative aspect to the sun, I guess it would be a relationship where you don't allow each other to "be yourselves"- where you hamper each other's individuality, and I guess growth (which you mentioned). Even though it is a positive aspect in your case, it's good to check out what a negative influence would do because anytime two planets are interacting there is a general similar energy involved, and even a positive can turn ugly if we're not careful. Well that is my take on it, here is what astro.com has to say:Composite Sun trine composite Saturn operates as a positive restraining influence on your relationship, giving it structure and form. Eventually you will know exactly what to expect from each other and what you must give. At the same time, this aspect provides that through your relationship you both will learn a great deal about yourselves and each other.
Although your relationship will not be very flamboyant, it will have a quiet and sober dignity that others will admire, for this is a sign of stability.
As a couple, you will prefer peace and quiet and established living patterns. You may be reluctant to experiment and encounter new aspects of yourselves. But be careful not to let caution become timidity, or fondness for familiar patterns become rigidity.
This relationship should outlast many others that may seem more energetic and vigorous. Because the energies signified by this aspect are released slowly, you will be able to stay together for a long time, no matter what kind of relationship you have.