I have bought both Encarta and Britannica for years (EB in printed edition too: 32 volumes, 32.000 pages). This is my opinion in brief: Encarta is excellent in all aspects, but Britannica's authoritative text (sometimes outdated) makes interesting to buy both.
TEXT: Britannica is a superb encyclopedia of text (not in visual aid) since 1768 (you know: an article by Einstein and so on...). Contents in electronic version differs from printed encyclopedia (very large articles have been shortened). Britannica claims that it has more entries that Encarta, but this is a joke: articles like "Mexico" are only one (with a lot of subdivisions) in Encarta, while in Britannica subdivisions are unconnected, and you must "jump" from one subdivision to another, which is slow and very annoying, especially if you want to copy it in "WORD". Very often, the text is not updated.
On the other hand, Encarta's text is not bad at all. Most articles have the name of their contributors and their professions, works...: They are not "John Doe". You can find large fragments of literary works, literature guides, a lot of sidebars and thousands of quotations. "Encarta Africana" is included. The Pop-Up (double clicking a word) Dictionary and Thesaurus has sound for correct pronunciation (by the way, it can read aloud, with a robotic and ugly voice, a whole article). The "Translation Dictionaries" to Spanish, French, German and Italian must be improved, because they are minimal. It gives you a lot of "Internet links", even if you are not connected. With Britannica you must be "on-line" and it searches in an EB Web page.
In theory you can update Britannica over the Internet free for a year quarterly (4 times), but this does not work. Encarta can be updated free EVERY WEEK with new articles and additions or corrections to the old ones (until October 2005). With Encarta updating really works. Technologically is amazing to see the changes in old items.
ATLAS Britannica has not a real atlas; only a worlds map whose maximum detail are the States of USA. Statistics are very poor. Encarta's Atlas is like another encyclopedia, with a great detail (1 inch = 10 miles all over the world) and 20 varieties of atlas presentations (statistical ones can be counted by dozens). If you look at a geographical article (city, river...) you can see in a corner where it is placed and, with only a click, open the Atlas. In articles of cities, if you are on-line, you can see in another corner the weather of this place in that moment. If it is a USA place, you can read the latest news.
MULTIMEDIA: They say that "serious" or "adult" readers do not care about "pictures"; that multimedia is only for kids. I do not agree, because I think that, sometimes, "A picture is worth a thousand words". Works of art, anatomy, historical maps, diagrams... Encarta devastates Britannica with a lot of photos, paintings, drawings, charts & tables, animations, interactivities, videos, music and sounds, pictures, 2-D and 3-D virtual tours, 360-degrees views, timeline, games... It is not only the quantity and quality. It is the easy access you have to all the multimedia, and that text and multimedia are fully integrated. Britannica is not really multimedia. It has photos and videos, but they make the program slow and sluggish. They should edit an alternative version with only text, as they did with the first edition in 1995. It performed fast and easy in old computers.
INTERFACE AND PERFORMANCE: This is the worst side of Britannica. With Encarta you only have to type a word or the beginning of a word to see all the articles and multimedia that contain it. If Encarta does not find anything, it gives you automatically alternative spellings. Even if you write the name of a small village lost in any country, you see it in the atlas. If you need to copy text or pictures, the integration with Microsoft WORD is perfect. It has additional ways to find content, including subject or multimedia browsing, "related articles" and the standard A-Z method. The "Research Organizer" is very helpful too. Encarta's TEXT FONT is very clear (Britannica's...) and you can choose 3 sizes.
Navigating with Britannica is disappointing. I will only give you an example: if you do not know the exact and correct spelling of a name or word, it does not help you with similar spellings (unless you open a window and "battle" with it). As I said before, the program's performance speed is very slow and sluggish, and it must be dramatically improved. To go "back and forward" you do not find any icon and you need to open a "menu".... One "pro" for Britannica: they say it works with Macintosh.
INTERNATIONAL EDITIONS: Encarta has a lot in different languages. The four I utilize (United Kingdom, Spanish, French and Italian ones) are adaptations of USA version, which is inexorable talking about History, Geography, Literature and other topics. The MISERABLE thing is that articles that equally concern any human being (Health, Mathematics and the rest of Sciences) are a VERY RESUMED translation of USA edition that is, of course, the best of all. Why Microsoft follows such a policy?
I repeat my modest piece of advice: Encarta is excellent in all aspects, but Britannica's authoritative text (sometimes outdated) make interesting to buy both.