When I switched to a Mac, I looked at various alternatives to Noteworthy (which is PC-only) and didn't like any of them - if you want to write music onto a score, I don't think there's anything better. However, I've now switched to using BarFly with tunes in ABC notation, & wouldn't change back. It's quick and easy to use, and you end up with tunes in plain text which can be read by anyone anywhere (using an ABC program, an online ABC converter, or even (with a bit of effort) by eye). (See my recent Lord Allenwater post for an example.) You also end up with access to some huge libraries of tunes, which can be downloaded in no time flat since the files are so small.

Noteworthy does make it easier to doodle your way through a tune note by note - what I tend to do these days is work through the start of the tune on recorder & go from there. Also, I haven't seen anything for PCs which displays a score as you key in the ABC, which is what BarFly does. I'd be surprised if there isn't something out there, though.