surely there are advantages and disadvantages but basically they both do the same thing. they calculate the physics using numerical calcs. physx has great cloth and ragdoll support and many great games and unreal engine use it but unity don't support all of it's features. let's wait for 3.0 to see more support i think they'll support cloth and fluid and sof bodies in 3.0 currently you can just use character controllers and rigidbodies and vehicles. havok is older than physx but physx is a good tech too. even built in physics like unigine's physics engine are good. actually they have common features and the quality of your physics system is dipendent to your skills. some people talk about what you say because physx uses CUDA to make itself faster but havok uses openCL and tech like compute shaders and it's optimizations will work on both nvidia and ATI cards. on the other hand CUDA is supported only on nvidia cards. but hey don't worry most gamers has nvidia cards and nvidia change it's approach for physx if needed. it will support cuda and openCL/compute shaders in future i think (if needed).

It depends on your opinions. Havok is better than physx in some aspects and vice versa. Physx will certainly not die any time soon.

First off, you have to ask yourself if you can integrate Havok - weather it being Unity related or not.

As far as I know, Havok seems to be better suited to AAA (triple A) titles as the developers might want to use the other tools it offers. Saying that there are plently of AAA titles that use PhysX.

It is not possible to say that Havok is better than PhysX because of the fact that it comes down to what project you are working on, the size of your team and so on. There is no way to accurately determine which of the two are better overall.

The new PhysX APEX modules such as APEX Clothing will allow you to more easily create character clothing. It is possible to do right now with PhysX the way it already is, though this will make everything simpler.

It is up to you to make ragdolls behave realistically, it is not a matter of the physics engine.