In an effort to make this an answerable question, and not just an opinion poll, I'll ask it like this:
Are there any third-party reports that compare ATI's Stream framework to NVidia's CUDA framework (ie, not from ATI or NVidia talking themselves up)?
I do not know of such a comparison, but I'll add some personal experience to the mix. Maybe between the users here we can develop a decent comparison/pros/cons/experience/lessons.
NVIDIA:
Tom's Hardware has an article, ATI Stream: Finally, CUDA Has Competition (2009-06-15).
Such a direct comparison would require someone to build a computational test suite which is API independent and runs on both CUDA and ATi Stream. People have tried but no platform independent solution is out yet. May be one of the GPU benchmarking powerhouses can get this done. There are other projects such as MAGMA or GPUlib which are blossoming under Nvidia tutelage. The world of GPGPU is very partisan indeed.
From a software perspective, it is not an apples-apples comparison since the programming models of the two are very different,
An application may map efficiently to Brook+ and not to CUDA or vice-versa. Optimum usage of hardware by writing good code makes a big difference