Harmonix- Finite elemente ..?

I would add Grand Prix Audio to your list.

Finite Elemente Ceraballs are indeed excellent (even great in their price range) and widely effective. I would advise anyone using a Spider rack to substitute the special Spider version of the Ceraballs for the resonance dampers that come customary with the rack. They are a remarkably effective and a worthwhile upgrade.

Harmonix on the other hand can be awesome! I am using RF-999 MT tuning feet under the spikes of my loudspeakers, expensive but in this case the right thing to do and with immediately audible effect. However, the important words to notice in the previous sentences are "can", "tuning", "in this case". Harmonix consider themselves a tuning system. Never just buy these products on the strength of what you read in the description. Talk to a knowledgeable dealer (they are very serious about this, I know of regular workshops in the Netherlands on their use) and try, the result just might leave you speechless.

Pricewise between the two and my own hot general recommendation are Aktyna ARIS, the second generation Evo2 is now on the market. One problem may be these are not widely available, I couldn't find a distributor in the US for one thing.
http://www.aktyna.com/PRODUIT/PRODUIT.php
Only in French, I'm afraid, but even if you can't read it, click "Accueil" and look at some of the prestigious brands they work together with ("partenaires")