An expert-led investigation carried out by Nature ...revealed numerous errors in both encyclopedias, but among 42 entries tested, the difference in accuracy was not particularly great: the average science entry in Wikipedia contained around four inaccuracies; Britannica, about three.
This comparison helps raising the prestige of Wikipedia on the account of lowering the prestige of Britannica. For the reader it is not the most practical comparison since most readers don't use Britannica, while Wikipedia is the 37th most visited website.
Since Google and Yahoo are drawing results from Wikipedia - a much more practical comparison could be drawn between a certain Wikipedia result and the other results. IMO the result from Wikipedia will be better than the top ten in 99% of the cases in any query in any field.
What Wikipedia and Britannica both lack is "gossip" – for example, an article about a famous artist will not include details about his next exhibition. This piece of information may be very relevant for the reader. This problem is going to be solved by QTpedia.