Question about MSI GeForce GTX 1080 GAMING X 8G

the 1080 because the GTX 1060 only has 6GB Vram and the 1080 has 8GB Vram and that is almost the most important thing and in addition not all games are SLI capable (the bigger ones are, like e.g. The poet GTA V etc.).

The 1060 does not have a classic SLI interface and must share its data via the PCI bus.
Only a few games support this way of connecting several graphics cards.

The performance in the few games that do support it is similar to that of the GTX1080.