Xbox One S is just a moniker for XB1 Slim. Same sh!t, different bucket, slightly more grunt under the hood.
Where things will get a bit more complicated in Xbox Scorpio, which is more or less XB1+. I imagine online MP will still work because the difference will be similar to Medium settings (XB1) vs Ultra settings (Scorpio) on a PC.
The core game will be the same on both consoles (they're both in the Xbox One family), but we're likely to get 60FPS on Scorpio and 30FPS on XB1 / XB1 S, which is a distinct advantage.