If you’re testing software that accesses device drivers or hardware, for example, a dual boot is probably going to be better. The only comment I’ll make about dual boot versus virtual machine is that a dual boot install will be a bit cleaner and faster, if that makes a difference. Even better, you can grab a free ISO disk image of Ubuntu directly from their Web site and once you’ve grabbed the 1.5GB file, it’s rather astonishingly easy to go from download to running Ubuntu installation!
Great choices all around, Ubuntu is my favorite Linux distro and VMware Fusion has been a cornerstone of my Mac experience for many years and does a great job installing and running just about every flavor of Linux (and Windows too).