Installing Fusion 4 is surprisingly simple: You just drag and drop the program to any directory you wish. There's no installer to run, and you can store the program anywhere. When you first launch Fusion, it asks for your administrative password and activates its extensions. But those extensions aren't hidden away in some low-level system folder where you'll never find them. Instead, they remain within the Fusion application bundle and automatically activate on subsequent launches.
More importantly, they're deactivated when you quit Fusion. In fact, when you quit Fusion, unless you choose to leave the Windows applications menu item in your Mac's menu bar, absolutely nothing Fusion-related is left running. This setup also makes uninstalling a snap—just drag the app to the trash, and you're done. Taking a program as complex as Fusion, and making it as easy to install and uninstall as any simple utility, is a major accomplishment.
Parallels, by contrast, is installed via an installer, its extensions are installed in the System folder and are always present, even when Desktop isn't running. In addition, two background processes continue to run after you quit Parallels. These processes don't take much RAM or CPU power, but they're there.
Preferences and virtual machine settings
Both of these programs have lots of settings options; Parallels Desktop has more of them and, consequently, has a more complicated preferences screen. Both of their preferences panels are reasonably well organized, doing a decent job of categorizing the various settings. One thing I don't like about Parallels is that it automatically enrolls you in the company's Customer Experience Program, which collects anonymous usage data; you have to opt out by disabling it in the Advanced section of Preferences. Fusion offers a similar program, but you have to opt in, not out.
When it comes to changing the settings for a virtual machine, the two programs take a slightly different approach: Parallels Desktop uses a floating window that's independent of the virtual machine being configured; that makes it easy to toggle between the settings and the virtual machine, but it's also easy to lose track of the settings window if you click another window to the foreground.
Fusion, by contrast, dims the virtual machine, and presents a fixed window in the center of the screen, on top of the virtual machine. Its settings window mimics that of System Preferences, while Parallels uses a tabs-and-lists layout. Some users may prefer one over the other, but I find they both work reasonably well.
Advantage: Neither (or both).