You really want to run OS X on the hardware, to take advantage of all the details of the various hardware nooks and crannies (power management, wireless hardware, etc.)

There's no real downside to running any Linux distro in a VM. (Again, I recommend VMware Fusion.) You can put each VM in its own full-screen space and switch around between them (and Mac OS X) with a keystroke.

Highly recommended.

(BTW, the way that Fusion works it that it essentially goes "under" OS X, when Fusion is running; it becomes a hypervisor, running OS X and the other OS(es) as virtual guests on an equal footing.

I heard a great talk at C4[1] from one of the VMware Mac engineers, who said that the Apple engineers weren't very happy to learn the details of how virtualization works (since it replaces their precious OS at the lowest level), but they didn't really have any choice, either. ;-)