It's probably wise to check the state laws about what is required. I suspect they vary by state. 

I follow what my CA inspectors taught me:

2025 Code: smoke detector in every bedroom, CO detector on every floor (level), so we use combo detectors in every floor's hallway as an extra measure. We just inspected a 3 story house with 5 bedrooms, so there were 5 smoke and 3 combo CO/Smoke in that house. We use Kidde brand too.

I have no horse in this race. My home is hard-wired because it was built that way. Beyond that, I say do what local code requires.

I have heard that the best way to go is hard-wired and combo, but I think a home inspector would likely have a more fact-based answer based on their experience, haha.

Sounds like J.R. Schloemer has done his homework, while I have not paid the attentions I should. Wake-up call for me.

Combo, I got the same brand you did, battery, at least one for every floor. I would not put one in the kitchen but not far from it either. 

I would do a CO monitor, hardwired with battery backup around combustable air units (but not too proximate to the units). Smoke I'd follow NEC and local fire ordinances, hardwired with a battery backup. Photoelectric as well as Ionization units if you can get them, & place them in the appropriate locations (at least 3 foot from a wall on the ceiling, preferably highest point).
Location per NEC is one on each floor, in each bedroom and outside of bedrooms, interconnected...in a nutshell and not an exhaustive list, so each house is going to have its own individual location layout. I am not so worried about "smart" detectors, just ones that work as needed.
I like Kidde & FireX personally, but I can't take anything away from First Alert, BRK, Nest, & others. I am just typically purchasing contractor packs for installation and can't get those brands in bulk like I can Kidde & FireX, in my area.
If there are any questions, contacting the fire department, they can tell you the best way to go about things, and possibly even help with placement and installation (our FD used to assist homeowners).

Combo battery unit operated PLUS use velcro to attach

They come with 10 year batteries now.  I think those are fine if the house isn't wired.

10 year battery with combo or hard wired if you have the high up wiring. Smart ones are not helpful. In July these are FEDERALLY regulated as to where they are required. No longer state by state. both smoke and carbon monoxide detectors should be placed inside each bedroom, outside each sleeping area, and on every level of a dwelling unit, with specific placement guidelines to ensure proper detection; smoke detectors should be installed high on walls or ceilings, while carbon monoxide detectors should be located within 15 feet of sleeping areas and fuel-burning appliances. 

Maryland passed a law in 2018 that you must have a hardwired non tamper proof battery smoke detector that is replaced every 10 years

I also use Kidde

in condos we have hard wired soke alarms, battery backup smoke alarms in each bedroom.  Separate carbonmonxide 10 year battery as recommended by code inspector when we renovated 5 years ago.

I would do the Kidde brand combo you have listed, but check to see what the rules are for your state.

In my area it's required to have a smoke detector within 15 feet of every bedroom.  So if there are three bedrooms clumped together... one outside of the three entrances would be adequate...

Just for the sake of clean lines and symmetry, I like the use of battery powered multi-purpose (CO and smoke combined) in one.  One of the things that most people forget with hard-wired detectors is that they still have a battery that requires replacement!! 

For the minimum number of detectors & style, we go with the regulations. As far as type it is the Combo with voice. Whether battery or hardwire is up to the owner. Most new homes are hardwired.

Kidde combo smoke and carbon are what I use when adding to locations where required. This is common in older homes. Rarely are they wired so we go with battery. 

Our house came with wired ones, they died.  We replaced with battery, not smart, they are fine.

I hate the new "10 year" ones.  So far I've yet to see one last that long.  Most of the ones I've had have died after 5 or 6 years.  They're required here in CA and I wish we could get older individual ones.  They just worked so much better.

Hardwired is best if you can, but they still need a battery backup for when the power goes out.

Linked ones are bad because when one starts chirping they all do and good luck finding which one it is.

I can't see a reason to have a phone app for these.  Integration into a monitored alarm system is better.

If this home is a keeper, check with a professional.  go with your gut on how much protection you need/want.  

My son is in charge of keeping these up to date in all of our homes. In the rentals they seem to need replacement much more frequently than they do in our home or offices

I just re-wired an older 1951 home and put in hard wired smoke detectors in all bedrooms and combo detectors in the hallway outside the bedrooms and in the room adjacent to the kitchen where there is a carbon monoxide source.  Hardwire is best if you can. Seems like First Alert and Kiddie are the two major brands out here in California.  Smoke detectors should be in all bedrooms and combo detectors should be outside all sleeping areas and on every floor and near all potential sources of carbon monoxide. This could include laundry rooms, basements, garages, water heater closets, etc. The combo detectors should be close to kitchens but not in the kitchen because too many false alarms could cause people to get frustrated and disable or remove the detectors which defeats the whole purpose.  

Fred Griffin Florida Real Estate if I were doing it for my home I believe in the KIS theology. I want something that is simple and easy to replace.

Depends upon YOUR STATE'S LAW...

• No direct federal law:

  There is no single federal law dictating the exact placement and type of smoke and carbon monoxide detectors required in homes. 

   
• State-level regulations:

  Individual states typically establish their own laws regarding smoke and carbon monoxide detector installation, often aligning with NFPA recommendations. 

   
• NFPA standards:

  The NFPA provides detailed guidelines on smoke and carbon monoxide detector placement, including recommendations for new construction and existing homes. 

   
• HUD compliance:

  The Department of Housing and Urban Development (HUD) does incorporate the International Fire Code standards regarding carbon monoxide detection only in properties they regulate.

I like a combo…that way you have battery back up if your power goes out! But I do like the synchronized fire alarms that are getting installed now!

Often state or local laws are more strict than Federal, so it's good to check. Smoke and carbon monoxide detectors are definitely important for safety and insurance reasons.

The combo is highly recommended and following the building codes is where they should be placed.

All new construction homes have hard-wired w/battery back up with both. If the home offers natural gas or propane for heating elements, you would want a combo unit to provide the highest level of safety.

I'd use the ones that now come with 10 year sealed battery. Also, make hey are connected, maybe by antennae in older homes, so if people are sleeping upstairs, and alarm goes off in basement, all alarms in the house will go off. https://www.loudoun.gov/819/Smoke-Alarms