Comparing Unity3D to O3D is a bit like comparing the Unreal Engine to OpenGL. Unity3D is much higher level, and provides lots of things like Ageia PhysX™, etc.

On the other hand not having to install a plugin would be a killer, so we are definitely keeping an eye on WebGL...

Unity3D is much more mature and full featured right now (and even more when we started working on this). We are looking into WebGL too, to soon for prime time but promising.