Fiskar :
What is the difference between Rampage VI Extreme and WS X299 SAGE?
The WS x299 Sage has 2 PEX8717 chips which allow 4 GPUs to run at 16x (e.g., x16/x16/x16/x16) with a CPU with 44 PCIe lanes, the Rampage does not (with 4 GPUs, it runs at x16/x8/x8/x8)
The WS x299 Sage has 2x1G Ethernet, the Rampage has 10G
The Rampage has a lot more bling than the Sage.
You can actually buy the Rampage (the Sage is available at a couple of on-line places, but only at gouging prices).
There are reviews of the Rampage; there are no reviews of the Sage yet.
If you need a serious work-station, and intend to use 3 or 4 GPUs for compute (not games), then you probably want the Sage. If you want something for gaming, then you are probably better off with a Coffee lake z370.