GP is right, Carbon Monoxide falls to the bottom, so detecting there is of great essence. Carbon Dioxide floats, so its detectors should be placed up.
NOTE: I meant Smoke detectors.
The relevant specific gravities (higher == more dense): Carbon dioxide - CO2 1.5189 Carbon monoxide - CO 0.9667
Perhaps the CO is generally produced mixed with other more heavier gasses and is colder, thus less conveyant.
I can't find source but I found a pamphlet that mentioned that when in fire you shouldn't try to crawl on the floor for risk of CO poisoning, or something.
If you're in a fire, stay low and get out. Source: I'm a firefighter.
There's a different UI for combustion gas analyzers, most any HVAC guy owns one if he does furnace work. They're about an order of magnitude more expensive and the ratio of CO2/O2 tells you quite a bit about the level of draft (and they usually have a CO sensor too which only comes into play when draft/mixture is tremendously rich)
Low-O2 detectors are also common, and approach the problem from the other direction. They're more useful for N2 or other inert gases, since you're less concerned with toxicity and more with plain asphyxiation. IIRC they're used in some places to enforce "No riding in the same lift as the liquid-N2 dewar" safety rules.
Everything I've seen shows that it generally tends to mix with air or rise slightly due to its slightly less dense weight, and all the sources I found say you can either use the ceiling or wall.