[zamalek]

To briefly answer your question directly: until a while back Havok had no GPU acceleration, where PhysX had GPU acceleration on NVIDIA cards only (best-effort on CPU for other GPUs). It made sense to always use PhysX because it was comparable to the competition on other GPUs but always better on NVIDIA.

It is now likely better to always use Havok FX (which runs on many GPUs), but PhysX had a time lead resulting in it being entrenched as well as being more mature.

The PhysX SDK is also free with immediate download[1] (causing indie devs to more likely use it and blog tutorials about it). Havok has a price tag, but also has a indie program called Strike that has an undetermined (inquiry-walled) price tag.[2]

[1]: http://www.nvidia.com/object/physx_downloads.html [2]: http://www.havok.com/havok-sales/