At this point in time, I''m not sure it matters. I run on IBM (Windows 98). I've experienced few problems an I assume that Finale runs as well if not better on newer version of the MS operating system. I have a frind in town who swears by Mac, and uses Finale. It looks the same, and I didn't recognize any real differences in the way the program performed. I think that the difference is not in Finale per se, but what you intend to do with it and what other programs, hardware or software you intend to use with it. For example, I use Finale just to compose and publish. I'm not using it for any MIDI features, or in conjunction with any other device or program other than my input device. My friend runs a small recording studio in town, and uses Finale to make his arrangments and charts, and uses it to drive MIDI input. He sees a real difference between this application on Mac and IBM, mainly because he does not have to mix and match IBM and Mac applications.
Other than that, I think it's really what computer you're comfortable with or already own. The program takes a lot of computing power, but I think that IBM/Microsoft have largely caught up to Mac on this issue. In the old days, it was Mac all the way.
