Schubert sounds more light, closer to Mozart.

Beethoven sounds more romantic.

Only Schubert has piano songs.

Only Beethoven has opera, piano trio, missa, violin concerto, piano concerto. (Beethoven, Schubert)

Beethoven sounds more repetitive and serious.

Mendelssohn usually sounds more light, romantic.

Only Beethoven has messa, string quartet, piano concerto, violin sonata (but Mendelssohn has piano trio).

Only Mendelssohn has organ. (Beethoven, Mendelssohn)

This is a nice starting island. But some of composers will be harder to guess. (JS Bach, Beethoven, Chopin, Debussy, Glass, Mozart, Pérotin, Rachmaninov, Tchaikovsky, Vivaldi, Wagner...)

Basically, Chopin is a poet and Beethoven is a warrior. Beethoven's Diabelli Variation appears often (probably it occupies a large portion of the database). Although such late works of him contain rich drama and tension which may trick you into into choosing Chopin, they can still can distinguished from the latter by their Beethovenian solemnity and willpower, dry texture and grey harmony, whereas Chopin is always sentimental and poetic, with his delicate grace notes and bouncing up-and-down development of melody (much funnier and fancier than Beethoven's), fluid texture and colourful harmony. (Beethoven, Chopin)

Very interesting island, having two well-known composers. Mozart is classical, but very melodic. Beethoven is romantic, but very structured and most of his melodies are simple and developed from previous melodies. Chamber works can be most difficult to guess. (Beethoven, Mozart)

Beethoven is more romantic and expressive then Haydn. Haydn sounds more repetitive and has unexpected changes. Also, Haydn does not have piano pieces in the game. (Beethoven, Haydn)

Beethoven - between romantic and classical, strict but expressive. Dvorák - post romantic, singing melodies, popular symphonic. (Beethoven, Dvorak)

Being both Romantic composers, Beethoven is closer to classical music, being more strict and repetitive. While Chopin is tender romantic, expressive. Probably most difficult will be to compare their piano pieces. Chopin does not have vocal, chamber and symphonic works in this game, mainly piano and piano concerts. (Beethoven, Chopin)