While I know that glxgears is not a reliable benchmark, the following demonstrates the issue between nvidia-96 and nouveau. I have two machines with the same hardware, and both use an nVidia Quatro4 900 XGL.  Both are running as non-Unity (gnome-classic) & both are set to use compiz identical (I can easily rotate the desktops w/Ctrl-Alt-mouse & flib between w/Ctrl-Alt-rightarrow etc). The difference is that one is running 11.04 w/nVidia-96, the other is 12.04 w/nouveau. Running glxgears on each one results in:

Glxgears on the on running 11.04
glxgears -info
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
GL_RENDERER   = Quadro4 900 XGL/AGP/SSE2
GL_VERSION    = 1.5.8 NVIDIA 96.43.20
GL_VENDOR     = NVIDIA Corporation
GL_EXTENSIONS = <snipped for brevity - but happy to post if helpful>
6578 frames in 5.0 seconds = 1315.524 FPS
7383 frames in 5.0 seconds = 1475.597 FPS
7594 frames in 5.0 seconds = 1518.743 FPS
7321 frames in 5.0 seconds = 1464.091 FPS
7351 frames in 5.0 seconds = 1470.171 FPS
$ sudo lshw -C display
  *-display               
       description: VGA compatible controller
       product: NV25GL [Quadro4 900 XGL]
       vendor: nVidia Corporation
       physical id: 0
       bus info: pci@0000:01:00.0
       version: a3
       width: 32 bits
       clock: 66MHz
       capabilities: pm agp agp-2.0 vga_controller bus_master cap_list rom
       configuration: driver=nvidia latency=248 maxlatency=1 mingnt=5
       resources: irq:16 memory:fd000000-fdffffff memory:d0000000-d7ffffff memory:dc780000-dc7fffff memory:fe9e0000-fe9fffff

Glxgears on the on running 12.04:
$ glxgears -info
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
GL_RENDERER   = Mesa DRI nv25 x86/MMX/SSE2
GL_VERSION    = 1.2 Mesa 8.0.2
GL_VENDOR     = Nouveau
GL_EXTENSIONS = <snipped for brevity>
4005 frames in 5.0 seconds = 800.934 FPS
4248 frames in 5.0 seconds = 849.541 FPS
4240 frames in 5.0 seconds = 847.968 FPS
3910 frames in 5.0 seconds = 781.843 FPS
4242 frames in 5.0 seconds = 848.253 FPS
$ sudo lshw -C display
  *-display               
       description: VGA compatible controller
       product: NV25GL [Quadro4 900 XGL]
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:01:00.0
       version: a3
       width: 32 bits
       clock: 66MHz
       capabilities: pm agp agp-2.0 vga_controller bus_master cap_list rom
       configuration: driver=nouveau latency=32 maxlatency=1 mingnt=5
       resources: irq:16 memory:fd000000-fdffffff memory:d0000000-d7ffffff memory:dc780000-dc7fffff memory:fe9e0000-fe9fffff

Quite a difference in performance between the two.

 That said: I fully realise that Ubuntu can't release an updated nvidia-96 until nVidia does. And from following this nvidia forum thread, it doesn't look like it will be anytime soon:
<http://www.nvnews.net/vbulletin/showthread.php?t=179489>
  <http://www.nvnews.net/vbulletin/showthread.php?t=179489&page=6>
So for those of us that require nvidia-96, my suggestion is to follow the nvida forum thread instead of creating a forum here.