I haven't used Notation, but I've used some of the other software mentioned, so I'll share my experience.

Musescore is completely free, relatively easy to learn, and makes really good looking printed scores. Downside: it is more buggy than you'd tolerate from a paid program, documentation is inadequate, and the sound file function is limited. The free sound font that comes with it is adequate but not really good enough for professional level work. You can install other sound fonts, but it's a little awkward to do, and unless you're willing to spend money, you probably aren't going to get much better ones. Also, Musescore can't generate mp3 files directly (though I believe there are plans to add this ability in the future); you have to save your Musescore music as a .wav file, and then use some other software to convert the .wav to .mp3. To be fair, Musescore really is intended as an application to generate printed scores, with the player something of an afterthought. Musescore is good for quickly generating fairly simple demo sound files, but isn't in my opinion good enough for producing professional level ones.

Finale has a cluster of related products, of which I've used the demo version of Finale and the demo and paid versions of Songwriter. Finale is very expensive, very hard and long to master, makes great looking scores, and can produce professional quality sound files which can include just about any sort of performance dynamics you want. If you are composing music to sell (whether for entertainment, commercials, movies, or video games,) Finale will do the job. If that's not what you are doing, though, I don't think Finale will be worth the time and expense.

Songwriter is one of several stripped down versions of Finale. It is much cheaper, easier to learn, makes equally good scores, and almost as good song files. Songwriter's sound font (if I remember correctly) is more limited, and you have less control over performance dynamics, but you can still get startlingly live-sounding files from it.

You can get free demo programs of Finale and Songwriter. If you buy Songwriter, you can get the price back in the form of a discount if you decide to upgrade to Finale.

Jon Corelis
Euripides' Hippolytos: A performance version with music