Tom Phillips spake thus:

OK, let's see: first of all, just the raw numbers:
Mozart wrote about 50 symphonies (41 of which are numbered and more or
less well known), while Beethoven wrote 9. (Of course, Beethoven's were
generally on a larger scale than Mozart's, especially the 9th, which is
closer to an oratorio in structure.)
Piano concerti: Mozart wrote 25 to Beethoven's 4. Violin concerti: the
score is 6 to 1. This exhausts Beethoven's concerto output, but Mozart
also wrote concerti for just about every conceivable instrument:
bassoon, horn, clarinet and flute, as well as his /Sinfonia Concertante/
for violin and viola.
Operas: Beethoven wrote just one, Fidelio, while Mozart wrote many,
including the incomparable /Don Giovanni/.
Now, in chamber music, Beethoven's output approached Mozart's; Beethoven
wrote two sets of string quartets (the op. 18 and op. , totalling 16
quartets) to Mozart's 23, plus any number of smaller works, including
piano and violin sonatas (and to Beethoven goes the prize for the
exquisite /Romance/ for violin and orchestra).

But of course raw numbers don't count, and certainly can't be used to
mark one a "better composer" than another (an inane and fallacious
concept if there ever was one); in that case, Wolfie would clearly have
Ludwig whipped. Actually, the grand prize for symphonies goes to neither
one, of course: the winner would clearly be Haydn, with his 104+
symphonies, each on a scale similar to Mozart's. And did you know that
in his entire oeuvre that Haydn never repeated a single theme? Not once
in all those hundred-plus symphonies, most of which are regarded as the
crown jewels in the genre. So if I were awarding the prize, in this
category Franz Joseph would get it.

How about this concept: comparing Beethoven to Mozart is like, well,
like comparing apples to oranges. Even though there are remarkable
similarities between late Mozart and early Beethoven--who was still a
Classical composer, despite the unmistakeable "romantic" nature of his
music--they were two entirely different entities, both musically and
temperamentally. Beethoven, of course, was known for his "Titanic"
qualities: if you knew anything about music, you'd recognize this in the
rhythmically driving, almost maniacal passages from the last movement of
his 7th symphony. Such writing would have been unknown to Mozart, who
preferred to infuse his music with emotion in more subtle ways; witness
the dark quality of his 24th piano concerto, which exploits the quality
of the key of C minor to rival anything of Beethoven's, although on a
smaller scale and without the crashing bombast. Likewise his Great Mass
in C minor, a piece unfortunately overshadowed in our time to his (also
great) Requiem due to a certain motion picture; listen to the opening
Kyrie and tell me that Mozart wasn't every bit as emotionally compelling
as Beethoven in this piece, with the strings striving ever upward.

I'll wager I know a hell of a lot more about music than you'll ever
know. Speaking of Beethoven and Mozart: have you ever played the /Missa
Solemnis/? Mozart's /Exultate Jubilate/? The Beethoven symphonies,
including the 9th? Any of Mozart's symphonies? Do you know that there
are actually words to the 39th symphony?[1]

Not to mention Brahm's /German Requiem/, his 4 symphonies (interesting
how the numeric output of symphonies declines from Haydn to Mozart to
Beethoven to Brahms, isn't it?), the Berlioz /Requiem/, several of
Haydn's major works, including his /Missa in Tempore Bellis/ (Mass in
Time of War, a piece that ought to be played daily for those in power in
our age) and /Creation/, Handel's /Messiah/ (no "the", please), and a
hell of a lot more. Come back and argue when you've got that under your
belt.

So before you go shooting off your mouth uttering inanities about who's
a better composer, save yourself the embarassment. They're both great,
and that's as much as most folks who know about this stuff will say.