BootMii can be installed as an IOS or as Boot2. If you can install it as boot2 (usually possible only on older Wiis) then this is the preferred method - BootMii as boot2 is nearly perfect brick protection. Starting up in BootMii mode lets you skip directly to the Homebrew Channel, ignoring any problems or bricks you might have. It can also back up your NAND (System Menu, all IOSes and saves) and restore it if something goes horribly wrong.
Priiloader is an update to Preloader since the original project is no longer developed. Priiloader does the same thing as Preloader: installs with your system menu to offer decent brick protection as well as enabling many helpful hacks (region-free games, unlocking savefile copying, skipping Health screen, etc). Priiloader is not as bulletproof as BootMii since many things that can mess up your System Menu (uninstalling the System Menu IOS, for example) will affect it as well, but it's still very handy since it lets you reach the Homebrew Channel to fix minor system issues or banner bricks.
You can have BootMii boot2 and Priiloader installed at the same time without any problems.
If you can install BootMii boot2, DO SO. If you can, Priiloader is optional but you will probably like the features. If you can't install BootMii as boot2, you really should install Priiloader.