I think it depends on what you use the camera to do. The famous reviewer Matt Granger made some video review on the two cameras. A7SII may be the primary choice for videography because of its 4k video shooting and the A7RII is the best choice for photography. the main difference is that the A7SII is optimized for full frame lenses while the A7RII is optimized for super 35 lenses.
I recommend the Canon EOS 1DX Mark II, the world's fastest DSLR, shooting at 14 frames per second (FPS) with full tracking autofocus and auto exposure for each and every frame. The price just fits your budget. It is $5,999.