When comparing Lilypond vs Soundslice, the Slant community recommends Lilypond for most people. In the question“What is the best music notation software?” Lilypond is ranked 3rd while Soundslice is ranked 9th. The most important reason people chose Lilypond is:
Lilypond is easily the most powerful free/libre engraver, and arguably even more powerful than the commercial heavyweights Sibelius and Finale, mostly because it is set up as a Turing-complete programming language: it can be extended for any notation task whatsoever. For most common tasks this is of course not necessary (Lilypond has them built in), but whenever some feature doesn't exist yet, the user can just add it.
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Pros
Pro Customisable in every detail
Lilypond is easily the most powerful free/libre engraver, and arguably even more powerful than the commercial heavyweights Sibelius and Finale, mostly because it is set up as a Turing-complete programming language: it can be extended for any notation task whatsoever. For most common tasks this is of course not necessary (Lilypond has them built in), but whenever some feature doesn't exist yet, the user can just add it.
Pro Best Typographical placement of musical elements, most readable engraving
Although there is a learning curve, the output of Lilypond is superior in every way. Very little after engraving adjustment is needed and the typography is both beautiful and very readable.
Pro Free and open source, much effective for mass engraving
Lilypond is free, and open source.
Lilypond community is active and helpful.
Lilypond is much effective for mass engraving, by typing text instead of 1-button mouse clicking.
Pro Imports various sound files formats
There are converters that could convert from midi, encore (.enc), ABC music (.abc), MusicXML(*. mxml), etc.
Pro Very very lightweight
Even more lightweight than MuseScore, just need the binary, some libraries (that come preinstalled with almost all systems), and a text editor (even ed will do).
Pro Most stable software & files
Lilypond is quite stable as a software. As files are text files they're virtually incorruptible, the files could be edited with any text editor.
Pro Web-based
Nothing to download or install.
Pro Imports Guitar Pro files
Great support for importing Guitar Pro files, of which there are many across the internet.
Pro Imports MusicXML
Great support for importing MusicXML from other notation editors.
Pro Great for transcribing
The editor can sync with YouTube videos or MP3s, which makes for a very efficient transcription workflow.
Pro Free
The basic version is free, though there are paid features such as ability to add custom keyboard shortcuts.
Cons
Con There's a big learning curve at first
Con Even for fans of text-based languages, Lilypond is not easy to use
Lilypond's syntax is influenced by both LaTeX and Lisp, and one might argue that it combines the disadvantages of both: it is both inconsistent and verbose. For simple tasks, ABCjs is more convenient to write, whereas for big projects a proper AST layout would be desirable.
Con Little control over engraving
The sheet music is "responsive," meaning it wraps to fit whatever device you're on. So this isn't intended for creating notation for print publication — it is native to the web.