% DNA in tail. This shows a good linearity with dose of damage over a seasonable range. Tail length tends to reach a maximum at a low level of damage.
Tail moment (product of tail length and relative tail content of DNA) is a less informative parameter (harder to visualize), since it has no generally accepted units. If you read in a paper that comets have a certain tail moment, it tells you nothing about the appearance of the comets, whereas with % tail DNA you can immediately visualize them. It is often important to know the sort of level of damage being reported – for instance, to be sure that the assay is not at saturation level.
[Tail moment could be useful, if everyone used the same units, and calibrated their analysis system. Tail moment has dimensions – length x relative DNA content. So the appropriate units would be μm x %, or μm x tail fraction. Meanwhile, just use % DNA in tail!]
"FAQ by Andrew Collins, Gunnar Brunborg and Jonas Nygren, 2006, NewGeneris FP7-project"