Each type has pros and cons, so it will ultimately come down to what works best for your space. Hardwired models require professional installation, but don’t require an outlet for power. Plug-in models are arguably the most convenient to move or replace, but take up an outlet and often wind up in outlets near the floor, which makes for poor placement.  Battery-powered CO detectors can be installed anywhere and won’t fail when the power goes out. One could argue they require more maintenance, as you’ll need to replace their batteries. Then again, you’ll need to do the same thing with the backup batteries for plug-in and hardwired CO detectors, so it isn’t really much more of a burden.