That's an age old question. I may be a little biased because I've been involved with Afteburn development since 1997 and have been using it in production since. I also have a personal gripe with Cebas, but I'll try to be as straightforward and honest as possible.
In the early days, there was much different about the two shaded volumetric tools, but I think the lines have blended quite a bit since then. Many argue that Pyrocluster is at times faster while others argue that it's easier to get good results with Afterburn. Personally, I think it's more of a personal preference. PyroCluster is bundled as part of Pro Optics Suite, probably the best value in the MAX plugin market. In many ways I think these tools, in particular RLF, are required for any serious effects work. However, I think Afterburn gives you more flexibilty when you forget all of the components of Pro Optics Suite and concentrate specifically on PyroCluster and Afterburn.
I like Afterburn because it's an entire suite of volumetric tools with primitive and particle support. Many say it's too slow, but that's probably because they are using the default values which aren't going to be optimal for all scenes. In fact with volumetrics you really have to tweak a few parameters to optimize the engine. Of course with this comes a trade off of speed vs. quality. One of my favorite new tools is the Explode daemon which allows you to make cool firery explosion effects. http://www.particlefx.com/images/ClusterBo...Brazil\_0047.jpg
