Sibelius 5 and Finale 08 are notation programs. Their main purpose is to lay out music to be printed out for musicians. They have decent playback options and both include Garritan's Personal Orchestra (well, a version of it). They are extremely useful if you are writing music for large groups with many staves.
As far as composing in these programs, its really a preference. I've owned Finale for many years (since they days with the crappy midi playback) and I cannot compose in it for the life of me. I work my music out somewhere else and then notate it into Finale. If this is the type of work you do, go for either Sibelius of Finale. Sibelius is more user friendly but I find Finale has a few more notational features than Sibelius. But I grew up on Finale so its hard for me to see any reason to switch.
Now if you're looking for a style of composing where you record what you play into the computer (in a similar fashion to garageband), then I would suggest any of the Digital Audio Workstations for you. There's a billion of them out there but some of the highlights are Logic, Digital Performer, Pro Tools, Cubase, Nuendo, and Ableton Live. All of these programs are very similar and work well for composing via sequencing (playing/recording into the computer). Most of them also have notation sections, but none of them are even close to as in depth as Finale or Sibelius.
My recommendation is work with garageband (since it comes with every new mac) for a week after you get your IMac and see if you like the way it works. Then decide if you should go in the sequencing direction, or the notational direction. Also, a piece of staff paper and a pencil never hurts!
Good luck in your musical endeavors!