Thanks to my colleague Daniela Petrelli for alerting me to a short article in the journal Nature about a survey of quality in the articles in these two publications (summary: wikipedia has some mistakes, but then Britannica has some too). There is also a plea for more academic experts to contribute to Wikipedia.
Giles, J. (2005) "Internet encyclopaedias go head to head." Nature, (438), 900-901.
http://www.nature.com/news/2005/ 051212/pdf/438900a.pdf