I've had the Sapphire package on my system almost as long as we have had it. Its a pretty good package, although it's pretty damn expensive... I think we paid around 10k for it.
I did a quick check of the other two packages you mentioned, and it looks like they are all the same price range. They all seem to have some similar effects, but I noticed Speed 6 has some particle effects that would be a nice addition to the Smoke, since it doesn't have that capability out of the box.
I would recommend get demo versions of each package. I know GenArts and the Foundry will give you a free 30 day demo license, and I bet you can get a demo version from Speed 6 as well. I would talk to the person selling you the Smoke, and see if they can set you up with some demo versions of the plugins. That way you can try them all out, and figure out what is going to be the most useful to you and the type of work you do.
Not sure if you are aware of this but BorisFX have sparks that you might find of interest and they are a lot less expensive than competing packages from other vendors. We offer the Boris Continuum Complete package as a sparks option for Smoke with about 150 plug-in effects and one of the advantages to having this on your system is that you can swap preset files created on one platform with any other supported platform. By this I mean that you can save an effect preset from BCC AVX in Avid MC and then use that same preset in BCC Sparks on Smoke or BCC AE in After Effects or BCC FxPlug in FCP.