Ubuntu 14.04 vs. Debian 7.3 vs. Debian Jessie Preview
Users of Debian Jessie should right away see OpenGL performance improvements with the open-source graphics drivers. Debian Wheezy is on the very old Mesa 8.0 while Debian testing and unstable has Mesa 9.2.2, hopefully we will see Mesa 10 land in the Debian world soon since there's many feature and performance improvements to the Intel, AMD Radeon, and Nouveau (NVIDIA) graphics drivers.
The newer Linux kernel with updated DRM drivers also helps out Debian Jessie's graphics and gaming performance.