Series S horizontal or vertical: Which is best

As long as you have at least 4-6 inches of clearance on all sides, it's not in an enclosed cabinet, and the vents are clear, I don't think it matters as MS will have thoroughly tested it both ways and don't recommend any particular orientation.

If I had to make an educated guess as to which is technically better, then I'd say horizontally, just because heat naturally rises, so it's coming up from the APU and being sucked out vertically through the main black vent. But that is just an educated guess, and I can't see it making a meaningful difference, if any at all.

EDIT - Gamers Nexus did some temperature tests with a Series X and the difference between horizontal and vertical was 0.1-0.2 of a degree, so yeah; it'll almost certainly be similar with the Series S, in other words - it doesn't really matter which way you have it.

Thank you was thinking the same because of the vents at the side

It doesn't really matter which way you have it, just make sure you have plenty of space for ventilation.

horizontal is best, because heat rises. Vertical with the series X is best for this reason

Xbox One Original - Horizontal Only
xbox S - horizontal and vertical
XBOX ONE X - horizontal and vertical
XBOX ONE SADE - horizontal and vertical