The Seasonic VERTEX GX-1200 wins in 5 out of 9 comparable specs, with 2 ties. Its biggest advantage is SATA Connectors (200% better).
Ion+ 2 Platinum 860W wins in 2 specs (MSRP: 35% better).
Recommendation
Choose VERTEX GX-1200 if you want the best overall performance — especially sata connectors.
Choose Ion+ 2 Platinum 860W if msrp matters more to you.
