As much as lilypond is awesome, it really is NOT the same sort of tool as musescore. Lilypond's purpose is to create "beautiful engraved (printed) scores". Musescore is as much a compositional tool as anything else, although the (now-former) project lead does value the printed output too, and believes that Musescore is now better than lilypond in several ways.

LilyPond is much, much better suited toward writing music that doesn't exist yet and is changing as you invent it.

Because, as reikonomusha put it in the previous comment, "you can edit, insert, splice, overwrite, etc. to your heart's content in your favorite text editor", which makes the overwhelming difference. You don't see the notation as you're entering it, but you have a way to iterate on that.

However, it greatly helps if you're a developer or computer scientist, since you're working with a compiled markup language.

LilyPond isn't the answer for regular mortals.

LilyPond has sane playback. If the piece you've written is 3 bars long so far, and you play the MIDI file that pops out of the compile, you hear 3 bars and then it terminates.

MuseScore requires you to specify the number of bars before you compose and then gives you sheet full of that many empty bars. When you click play, it plays through all the notes you have composed so far ... and then doesn't stop! It goes through the empty bars. It's like the Forrest Gump of music programs. Run, MuseScore, run! If you put a double bar in its path, it doesn't get fooled: that's not the actual end of the file, so it blows right through it.