Power (energy released by the reaction per unit time) would probably be the best 'metric' for this.

Of course they also have the metric of kilotons of TNT (20 kilotons was the approximate payload of the Hiroshima bomb IIRC) and yes they also use the Hiroshima bomb as a metric (e.g. measuring North Korea's nuke), but they aren't really the best metrics in the world to use because it measures only the total released energy, which is nice to know, but doesn't tell you the rate at all. [e.g. 20 kilotons in a few seconds or less would be a big big explosion, but 20 kilotons over 5 decades is pretty negligible]

Edit: Oh, and it probably has to be normalised per unit volume too, if it's spread out too thinly it also is quite meaningless again

My final answer would probably power normalised by volume.

Edit 2: I also note that you seem to be referring a little to the yield of the substance (evidenced by the different materials you've talked about)

This post has been edited by bgeh: Jun 14 2009, 10:10 AM