I won't reccommend a brand .... while there ARE differences, there are too many other things going on; it seems that the source of supply is at least as important as the brand.
Hard wire with battery back-up? Gee, maybe someone will make that a code proposal
(It already IS code, axcept for existing dwellings).
Your manner of interconnecting them is perfectly fine. NEC aside, many locales do not require smoke alarms to be on AFCI circuits; check your local rules.
As for type .... "ionization" types are cheapest, and most readily available. They are fine for most locations. Most makers also make a "photoelectric" type, which is used where there are things that can cause false alarms for the ionization type .... things like the 'plume' of hot air off the furnace, cooking vapors from the kitchen, etc.
Personally, I would not choose a combined smoke / CO detector. I have two basic reasons for this:
1) The placement of CO detectors is nowhere near as 'settled' as is the placement of smoke detectors, and I have little reason to think they should be placed in the same locations; and,
2) CO detectors have a definite 5 year life, and will simply not work 5 years after you turn them on. Smoke detectors, by comparison, are 'estimated' to last ten years (for the ionization type), and a photoelectric type might last forever (yea, right!)
Another quirk to CO detectors is that they are limited to operating at what are essentially 'normal room' temperatures. They simply aren't suited for humid or cold locations.
