Do you have an HOA? Just asking because ours requires real slate.
Real slate would be my choice, even slightly more expensively, because synthetic slate is new enough I question its longevity under real weather conditions. Also, most of the savings would be in choosing synthetic while you are still building the roof-- real slate is heavy and requires more infrastructure. Once you have the roof in place, I wouldn't save 10% by using an untested material.
We are at the same spot and it is a major expense. But like I said, our HOA would be an obstacle to choosing synthetic, anyway.