http://www.sidefx.com/index.php?option=com_content&task=view&id=1009&Itemid=320
Q. What is the difference between Houdini Apprentice and Houdini Apprentice HD?
A. Houdini Apprentice HD does not put the Houdini wordmark in your renders. Further, for still images, your render size is unlimited. For animations, you are restricted to HD 1080p resolution - 1920x1080. Both Houdini Apprentice and Houdini Apprentice HD use the non-commercial .hipnc file format. Also, third-party rendering (e.g., RenderMan) is unavailable in both Houdini Apprentice and Houdini Apprentice HD.
So, Houdini FX commercial, No render size limitation, a commercial input/output format along with being able to input AND output FBX, etc. versus HD only able to input FBX but Input/Output .obj for example and you can render to 3rd party renderers. You also get additional support other than their website forum. I think that’s about it.
3rd party rendering is rendering outside of a software package’s native (in-built) renderer. The example given is RenderMan. So, You can only use Houdini’s Mantra renderer, which is very good.
Expensive?
Maya or Max @ $3675 + Realflow @ $2500 = $6175
vs
Houdini FX @ $4,495
I say, give a little here and take a little there, all round it’s considerably less expensive in comparison.