Think it'll partly be affected by the number of fans, which case and whether you use a high end GPU (which throws out a lot of heat) as well.
In my recent experience of messing around with AIO and Water and switching them around:
1. If you have a hot GPU, and not enough extraction for the GPU, then an AIO for the CPU is better (less heat sources in the rear of the case to cause a Hot spot/Heat Trap)
2. If you have a hot GPU, and plenty of extraction for the GPU, then either AIO or Air is fine. With preference on Air for ease of cleaning and worry free if pumps fail (although that is what the warranty is for I guess)
3. If you have a hot GPU, but not enough venting spots in your case for more fans to extract heat, especially near the rear back, then AIO is better (like 1, less heat source near rear of case to cause a Hot spot/Heat Trap)
4. If you don't have a hot GPU, and have enough airflow, then Air is better (CPU cooling is your only concern and the area in the rear back of the case will less likely become a Hot spot/Heat Trap)
5. If you don't have a hot GPU, but are short on airflow, then AIO is better (as there's less heat generating components inside the case to heat the inside up before the heat can be removed)
AIO and Air (just on their own, without any other factors) will cool as well as each other in my experience. Just noise, maintenance, etc are very different.