I'm a skeptic when it comes to synthetic underlayment. Its not a bad product, but the anecdotal evidence is not there yet. We won't really know the real data until probably the next generation. As a roofer, I prefer 30 lb. underlayment. It has served the roofing industry well throughout its years of use. Although not perfect, it will hold up under a properly maintained roof.
How the layers overlap on the roof, the folds the installer makes, the cuts they choose all are greater indicators of quality than a products material composition.
The price difference between synthetic vs 30 lb. felt is negligible in my opinion, and sometimes they are about the same depending on the size of the roof. This anomaly is caused by the different length in the rolls. It has always seemed strange to me that synthetic is often pitched as an upgrade, all branding and marketing I assume.