Ed Felten's doing some empirical comparisons of the online Britannica versus Wikipedia, and Wikipedia's doing pretty good!
Virtual memory: Wikipedia has a pretty good entry; Britannica has no entry for virtual memory, and doesn't appear to discuss the concept elsewhere, either. Verdict: advantage Wikipedia.
Public-key cryptography: Good, accurate entries in both. Verdict: toss-up.
Microsoft antitrust case: Britannica has only two sentences, saying that Judge Jackson ruled against Microsoft and ordered a breakup, and that the Court of Appeals overturned the breakup but agreed that Microsoft had broken the law. That's correct, but it leaves out the settlement. Wikipedia's entry is much longer but error-prone. Verdict: big advantage to Britannica.
Overall verdict: Wikipedia's advantage is in having more, longer, and more current entries. If it weren't for the Microsoft-case entry, Wikipedia would have been the winner hands down. Britannica's advantage is in having lower variance in the quality of its entries.