encyclopedias

I think both books are outstanding, and more complementary than rivals. In fact, I use both. Nevertheless, there are some differences.
Columbia's big dimensions and weight (8.9 pounds/4 kg) make almost necessary to read it on a desk. Britannica Concise (BCE) is 6.7 pounds/3 kg and smaller.
Both utilize an extremely small font size. Columbia contains 6.5 million words. BCE "only" 2.6.
Britannica C has over 2000 photographs, maps, tables, drawings, color illustrations; nations flags ... In Columbia, illustrations are sparse, limited to about 500 black-and-white line drawings.
Columbia's 6th edition stopped in 1999. BCE is of April 2003 and is edited every year in spring, but I don't know if they are going to update it or not.
Britannica Concise has articles like Super Bowl, Viagra, Coca-Cola, Big Stick Policy, Mother's Day and Father's Day ... that don't exist in Columbia.
Quantity does not always mean Quality. B Concise seems to be more shrewd, witty and, by the way, less subjective. For instance, Columbia's article Homosexuality concludes in this way: "But AIDS (.....) also sparked moralistic reactions; some felt, for example, that it represented a form of judgment on homosexuality". BCE's same article is shorter, but neutral, and does not say such a thing.
As I said before, both books are outstanding and complementary.

Let me give you an example of the great comprehensiveness of this volume...

Yesterday, I happened to watch a re-run of STAR TREK: THE NEXT GENERATION, the episode in which Picard talks about Fermat's Last Theorem, how it might never be proved. Remembering that it was proved some years ago, I looked up "Fermat's Last Theorem" in my BCE and saw that it had been finally been proved in 1994. Finding an episode guide for ST:TNG online, I saw that that particular episode had been first broadcast in '89. It was really cute to see that the theorem, first referred to in Fermat's notebook in 1637 (!!), was finally proved only 5 years after Picard was made to say that it might never be done.

(Moral of the story: "Never say never"!!)

This is a condensed version of the great 32-volume work. You can't have too many reference works that you never actually use anymore in your life (now that most of it is on the web), so I thought I'd check this out. What's one more weighty tome that I never read that sits on the shelf?

Well, that was my first reaction. Actually, this is a very nicely done book. It has 28,000 articles and many beautiful, color illustrations, and it's actually a pretty good book to just browse through and look at. It's comparable to the well-known Columbia Encyclopedia, which I have often used in libraries, and now in its 6th ed. It has many more entries, at 51,000, but it's not as concise either, but the Columbia work has suggestions for further reading. Both books are fine reference works, however, and should serve you well whichever one you decide to buy.