Last year Titleist tweaked the cores and mid-layers in its Pro V1 range to further improve performance. The Pro V1 acquired a softer core and reworked mid-layer for lower driver spin and increased distance; the Pro V1x got a new soft dual core to lower spin rates and bring more control on fuller iron shots.
I'd always opted for the Pro V1 over the Pro V1x, believing the latter to be the best fit for me. But I may have been converted. Regardless of swing speed, my spin appeared to benefit from being lowered as I found the Pro V1x long and certainly softer around the greens than its stablemate.