The U14 or D15 cost about as much as a 360/420 AIO.
Whilst they can have similar performance you aren’t saving on costs especially when you buy a 2nd fan for the main cooler which is another $25 or so if it’s Noctua.
I switched from a Noctua to a 320mm AIO and was not happy. The cooling capacity with the AIO is slightly higher, but it makes much more sound at idle. The pump never stops and unlike a loafing fan it makes a high-Q tone that is clearly audible in a quiet room. That was very disappointing. All the reviews said the AIO is “silent” which would have been easily disproven by objective measurements, even with free apps on a smartphone.
I'm in similar boat; my current desktop has 360mm aio cooling, and it makes me wonder what is peoples baseline if it can in any way be considered quiet. It feels louder than any my past aircooled builds, I definitely expected more from it.
Whilst they can have similar performance you aren’t saving on costs especially when you buy a 2nd fan for the main cooler which is another $25 or so if it’s Noctua.