"In my own opinion, the NH-D15S is the better cooler when compared to the NH-D15. The better expansion-slot clearance allows it to fit into far more builds, which gives it a wider appeal. That and the fact that users can still slap a second fan on for maximum performance without issue certainly doesn't hurt either. That second fan doesn't really matter on mainstream systems, but it would prove a bit more useful with AMD or Intel HEDT platforms. Noctua has definitely set a new bar in terms of what one can expect from monstrous heatsinks in that the NH-D15S is compatible with many mini-ITX and micro-ATX systems where the first expansion slot is PCIe x16."
Crazyeyesreaper - TechPowerUp.
Noctua NH-D15S CPU cooler review by TechPowerUp - Introduction & Conclusion.