Hardwired is nice, but you still need a battery backup for when the power goes out.

Hard-wired with battery backup.  

The battery only is a PITA when it comes time to change the battery. People say it is easy ... not at 2 am which is the time they are all set to start beeping. 

Maryland passed a law in 2018, It requires replacement of any battery-only operated smoke alarm that is more than 10 years old with a unit powered by a 10-year sealed-in battery. It's usually the first thing a home inspector looks for.

my preference is battery operated simply for ease of installation, easy to test, they are reliable and easier to replace, IMHO.

Most newer buildings will have hard wired. One advantage/disadvantage with hard wired is that when one goes off they all go off. Older properties may only have the option of battery. Regardless they should be changed periodically to ensure functionality.

I have both & the newest one is a combo which is better with C02.

In one of my condos I owned I was upset when there was a small kitchen fire & the hard wired detector never worked. Firefighters set it on fire with a lighter & it still didn't go off.  I've never trusted them since.

I was under the impression that most were hard wired with a battery back up.  I do know that these units do have a shelf life and need more than a battery replacement after a period.  @10 years.  not an expert but I did spend two nights in a Holiday Inn this weekend.  

Hard wire I feel they are safer and for those that will remove them, forget to test or check them every 6 months. There have been many home fires that had smoke detectors in the home with dead or removed batteries. We have hard wire with battery back up.

I had to pull out ALL my hardwired smoke detectors and put battery operated ones in . . . when/if I ever sell my house I'll put them back but with the dog, noooooo. they would all take turns freaking out and it was excruciating for her! 

I prefer the combo smoke/co units and battery op since now the batteries last 10 years. One pro for hard-wired is if one goes off they all go off.

I have no preference and have hard wired in the house with a newer battery smoke/co2 combo.

At the old cottage, it has two of the newer battery one.

Mine is hard wired but has a battery as backup.  I have no preference as long as it works. 

Mine are hardwired because that's how my house was built. They also use batteries.

Hard wired are now required by code in our area, so hard wired.

Battery. Why?
The comsumer grade detectors become so discolored by the simple degradation of the plastic that replacement is needed about 6 years.
Of course, accordking the the 'HardWired Council of America (HWCA)' more people are injured by uncontrolled decent of a ladder than those hit by a meteor.
So be safe and get it hard wired advises the HWCA.

Maryland has a law since 2018 ...no alarm battery-powered or hard-wired may be older than 10 years from the date of manufacture and if it's a battery, it must be sealed and tamper-resistant. 

Our system in hardwired and it works great.  I rarely see battery operated fire detectors in homes.

we have both.  

Fred,

Battery...A

hard wired because they communicate and if I have a fire in the basement I'm alerted all over the house

Hardwire with battery back up is common.

I wish they would get away from those overpriced 9v batteries.

In new construction it may be best to hird wire them, for old homes though this may be more difficult.

Hard wired with battery back-up is the way to go! 

Hard-wired with a backup battery. I just feel more secure with a stable source of energy, and an alterntive source if anything goes wrong. Since newer building codes call for it, I suppose that is the direction things are going.

I would prefer battery so if they go off when cooking the battery can be temporarily be removed.