I think both AV-C0mparatives and AV-Test are good testing organizations. In the past there were floating some "monkey sandwich" (internet myth) stories on AV-Test's questionable independent reputation. There is really no proof of these ancient allegations, only long, long, long, long, really long, I mean really really long ago ....
There was an issue between a British consumer association and an Anti-Virus company because the consumer association used according to the AV-company a faulty testing procedure and that AV wanted to prevent that consumer association publishing the results or wanted that they corrected the results or something.
After the initial "we are going to court" noise the whole incident ended with a sizzle (is that correct English?) and it seemed that in the pre-court mediation sessions AV-test assisted the AV with its argumentation and AV-Comparatives supported the consumer association. As far as I can remember it was a lot to do about nothing.
In the AV-Test vs AV-Comparatives, the latter has both its testing method
and test-lab certified by independent quality organisations (see spolier) and has a stronger relation to the academic world (see
link). When my memory serves me right AV-test method was also EICAR certified in the past and in the past also had a relation with the University of Magdeburg. AV-Test is the largest independent testlab so they probably have not lost their certification and academic relation, they just don't communicate it anymore (it is a proven and well known marketing tactic to stop publishing attributes/qualifications of your product and/or organization when the runner up has more impressive facts to publish on that same attribute/qualification).
www.av-test.org
