you can also use crimp-on connectors (bullet or spade types).

The only real advantage of these are that, if in the future you decide to change HUs for something that has a different wire harness, you can simply unplug the old HU's side of the harness and put matching connectors on the new harness and plug it in, without needing to buy a new harness for the car's side (metra/scosche).

Even though I don't plan on a new HU for the car (beyond the Kenwood), I like to keep my options.