In a single word ‘Both’.
To understand where to put your CO detectors at home or where to take readings in construction, a little science will help. Carbon Monoxide (CO) weighs slightly lighter than air with a vapour density of .97 compared to air's density of 1.
But here’s the catch, that weight is calculated at 1 ATM (Atmosphere) or neutral atmospheric pressure (neither high or low pressure, right in the middle of normal), which is 101.3Kpa in metric or 29.92 inches for imperial. A brief explanation of atmospheric pressure.... If you take a square inch column of atmosphere/air from sea level up to approx 50 miles in the sky, that column of air weighs an average of 14.7 lbs. On high pressure days it weighs more than 14.7lbs and on low pressure days it weighs less.
Therefore on days where the atmospheric pressure is at 101.3Kpa (14.7lbs) or lower, carbon monoxide will be allowed to naturally rise as it is naturally 3% lighter than air, therefore a detector on or close to the ceiling is most ideal for low pressure days. However you can’t forget the flip side, when we have a high pressure system above 101.3Kpa, the extra pressure from the atmosphere can and will usually force the carbon monoxide down, because it is only 3% lighter than air. Depending on how high of a pressure system you are under, the carbon monoxide can either; be held in place resembling a buoyant gas neither rising or sinking or it can be pushed down to floor level and other lower levels like basements, pits & excavations.
Therefore it is suggested; have a combo CO/smoke detector on the ceiling directly outside your bedrooms and also have a plug-in CO detector as low as possible near the floor in the same area. You are covered either way.
Do not install detectors in the basement due to false readings given off by gas furnaces.
Atmospheric pressure can change vapor density expectations on gasses with a density range between .95 - 1.05 (Air = 1)