I mean, even when maintenance isn't really different in either, still in case of AIO cooling I'm paying more for something which is reported to be 8dB louder than the cheaper air alternative?
No, in the case of
THAT AIO cooler, you'd be paying more for ... etc. Not in the case of AIO cooling in general.
You could pick a different air cooler and find the reverse no doubt.
It depends on the model of AIO. And what fan curves are being used. And the rest of the system.
For instance, the Fractal Cooler you're talking about may have better compatibility with cases or certain RAM kits, or perhaps a system with it would have different GPU thermals than the D15/S since it's effectively adding fans to the intake/exhaust system of the case. I've not looked at it, but I expect it has its place.
You're buying a 9900k, and potentially going for 'light overclocking'. The lightest OC you can reasonably do on a 9900k is 5GHz all-core - 5GHz being what it boosts to on 2 of its 8 cores at stock, anyway, so the OC is just bringing the rest of the cores up to the 1 or 2 core boost level. You'd probably want to try pushing past 5GHz to 5.1, maybe 5.2GHz.
On an open-air test bench with an NH D15S specifically, eteknix found their 9900k went to 84 degrees at 5GHz all-core.
The latest CPU from Intel is here at last, and it’s built to be a full-on fire-breathing beast when it comes to performance figures. Firstly, this is the counter-attack from Intel we’ve been waiting…
www.eteknix.com
Noting that this is an open air test bench, not an enclosed case.
Which is why for a 9900k, with an overclock, I'd start looking at liquid cooling over an NH D15S. And probably even over the NH D15.
In the GN review, the Arctic Liquid Freezer II kept a 3950x OCed (a fair analogy to the 9900k OCed n power consumption at least) 6 degrees cooler than the NH D15 (non-S) when noise normalised to 35 degrees. Which is difference worth noting.