15-Way Open-Source Intel/AMD/NVIDIA GPU Comparison
Due to the re-clocking situation for Nouveau, when it comes to open-source drivers as they appear "out of the box" on modern Linux distributions, the AMD Radeon Gallium3D performance is generally superior to that of the reverse-engineered NVIDIA driver. I hope that Nouveau's re-clocking situation will improve in the near term with Linux kernel improvements.
When both Intel and AMD hardware are running on open-source drivers, these Linux OpenGL benchmarks show the Intel HD Graphics 4600 "Haswell" running at roughly the speed of a Radeon HD 4670 graphics card on the open-source driver. Again, the Linux 3.9 kernel and Mesa 9.2 were at play.