The critical thing about the alarms is that they should have their trigger wire interconnected, so that if one goes off, they all do. The most reliable way to ensure that works is to use units from the same company. The trigger levels and type may not be compatible if you try to mix and match. I prefer hardwired with battery backup. If you don't have too many power outages, put a lithium backup battery in them, and it should last the 10-years the things are rated before blanket replacement is called for as expressed by the Fire Marshalls Association. Interconnecting them all with the trigger wire adds some wiring complexity, since it obviously needs to run all over the house. It's a good idea to have these on their own circuit so say a hair dryer doesn't trip the circuit and leave you without or running off batteries. Nothing more annoying than the low-battery beep from the things that starts in the middle of the night! As noted, though, if they are infrequent and not too long, the a/c powered units with battery backup, a good lithium battery should last the life of the thing, so this is minimized. I think I've seen some with a rechargable battery, but those don't last forever, either.
The eariler (10-15 years ago) CO detectors weren't all that great. Read the fine print on them, too, as the sensor may not last forever - 5-years may be close to the life on those, so combining it with a smoke detector would keep the clutter down, but you might want to replace them more often.
FWIW, if you can find contractor packs of the things, you eliminate the retail packaging, and can get them for about 1/2 of the normal retail pricing, even at the big box stores (HD does carry some in contractor packs).