I have had my PC in many places. You didn’t have an option for above your desk, which is where mine is. It is about Top of Monitor height. I have found that next to wall, the computer picks up the cool wall air and the temperature inside is cooler.
When the computer is on the floor, it picks up the dust from the carpet or floor when you walk. Yes, when you walk on the carpet it stirs up the dust in the carpet and gets sucked into the intake of the computer. Even with frequent vacuuming, a floor level computer will get two or three times dirty faster than a computer mounted higher. If you have to leave your computer on the floor, try and raise it up a bit by using bricks a cinder block or even just some old encyclopedias to get it off the ground