The new forums will be named Coin Return (based on the most recent vote)! You can check on the status and timeline of the transition to the new forums here.
The Guiding Principles and New Rules document is now in effect.
Reason is cool but it doesn't take VST plugins so you're a bit limited as far as that goes. I think the "Final Cut Music" you're talking about is called Live, which I have not used.
Cubase is available for OSX, and it is well-respected and I can personally vouch for it being great. I haven't used version 4 yet, and I've heard there are some issues with it, so you might want to check out the kvraudio forums before you decide.
If you're gonna spend money and you're gonna work with loops, you might as well pony up for Ableton Live. It's the bees knees if your music ends up hitting a "loop thing" at any point, and it keeps getting better.
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Also, I've heard FL Studio has a Mac version, but I'm not positive.
Cubase is available for OSX, and it is well-respected and I can personally vouch for it being great. I haven't used version 4 yet, and I've heard there are some issues with it, so you might want to check out the kvraudio forums before you decide.
Most of these things are pretty pricey.
EDIT: I haven't used it, but Tracktion is supposed to be pretty good:
http://www.mackie.com/products/software/index.html#tracktion2soft
New version soon.