If you must switch...go NUKE

NUKE is a better alternative if you feel like you "have to" switch and it runs natively under OS X. The thing is that I don't really think there's enough change from version to version of any compositing software over the last few cycles to justify change unless you're going into specialized software, like mokey from Imagineer systems. Apple is still working on Shake, what exactly they're waiting for I'm not sure, if I was a betting man I'd bet that Shake 5 will hit not too long after "Snow Leopard." The best thing any software developer can do to a piece of complex software is make it run faster and making the OS and all homegrown software massively parallel should certainly open the throttle a bit.