Wikipedias are much, much larger, and much more comprehensive. As your article evocatively points out, Britannica in print form occupies about two bookshelves, in about 32 volumes. A hypothetical printed version of the English Wikipedia was estimated at about 1700 volumes in 2010: http://en.wikipedia.org/wiki/Wikipedia:Size_in_volumes

So if we’re talking about gaps in coverage, Britannica unavoidably suffers enormous gaps in coverage relative to English Wikipedia, simply because English Wikipedia contains some 50 pages of material for every page in Britannica. As an example, the first example that came to my mind, compare http://www.britannica.com/EBchecked/topic/588941/tetrachloroethylene (126 words, of which 100 are available to anyone) with https://en.wikipedia.org/wiki/Tetrachloroethylene (900 words plus 13 references, two diagrams, and a table of chemical property data). They’re not even in the same league. The fact that the Britannica article appears to have not been updated in several decades, and so omits the extremely important information that tetrachloroethylene is a probable carcinogen, is comparatively insignificant!

Now of course we can’t expect English Wikipedia to cover chemical compounds to the same depth that it covers, say, Pokémon: https://en.wikipedia.org/wiki/Zoroark — I think that’s what you’re saying about “the typically parochial interests of its contributors”. But it still covers chemical compounds, and everything else I’ve compared, in dramatically greater depth than Britannica does.