The Seasonic VERTEX GX-1000 wins in 5 out of 9 comparable specs, with 2 ties. Its biggest advantage is 12VHPWR Connector (100% better).
Prime TX-850 wins in 2 specs (Efficiency: 50% better).
Recommendation
Choose VERTEX GX-1000 if you want the best overall performance — especially 12vhpwr connector.
Choose Prime TX-850 if efficiency matters more to you.
