mmH2O or CFM? Which is to look for a fan on AIO cpu cooler?

There isn't a whole lot of different between fans at a given amount of airflow. Most of what can be achieved by switching to a lower noise fan can also be achieved by adjusting your fan speed profile. The only major difference you'll see is if your fan has a lot of bearing noise (e.g. clicking or whirring that doesn't go away as fan speed drops). Moving air itself makes noise.

If you're pushing air through a radiator, be it a cpu radiator or an aio cooling one, or through a filter at the front of the case, ideally you want an air pressure fan because they maintain high air pressure, for quite a bit of distance behind the fan, rather than dissipating in all directions.

The trouble is this all doesn't really apply to you. The fans you link, the silverstone one has both higher pressure and higher CFM because it can reach higher RPMs. If you want to quiet it down, you need a utility like SpeedFan or something that comes with your cpu cooler to lower the speed of its fans.

However, this will naturally reduce the cooling. In the end it's always a trade-off between performance and noise. Trouble with your case is that it's very cramped, and as far as I can tell you've mounted the liquid cooler to the front fan, which itself is behind a solid cover with only side holes for incoming air, limiting its cooling potential a lot. All the sucking power of the fan in the world won't help if you've got pin hole sized side air intakes and so even though front panels are meant to reduce noise, they end up being just as loud cause fans ramp up to compensate not getting enough cool air.

Not sure what to tell you solution wise except that getting that noctua fan won't help. You'd achieve the same result just managing the RPMs yourself, with similar increase in temperature.

There is a version of your case where front panel is a mesh instead of solid. That might help sucking fresh air in and cooling it a bit but as I said previously, though that may help you ramp down the fans a little, now their sound will be unimpeded so I'm not sure how much improvement noise wise it'll be. Personally? If you want quiet but still not sacrificing cooling, you need a bigger case. There are many advantages to smaller cases, but noise/cooling generally aren't among them.

If you're dead set on changing fans, try some of the beQuiet! line and go for magnetic or sso bearing as they are supposedly a bit better sounding whatever that means, in addition to lasting longer. You can check on youtube videos when people test fans, what they're supposed to sound like under load so listen to it and judge for yourself.

As Sedivy says, cooling performance and noise are inversely related. You can make your current fan quieter by turning its speed down. You generally can't push more air without making more noise because it's the movement of air itself that makes the noise.

Hmm. Well do you have space for a second fan? I'd think that would be the biggest limitation. Otherwise, I don't think it can hurt, provided you match their rpms and air flow pretty closely.
Also, was this increase in temperatures sudden? Or was it gradual over time? If gradual, you might just need a thorough cleaning of all components, starting with your radiator. If it was sudden, you gotta wonder if there is an issue somewhere. Is your pump failing? Is the fan still running ok? Have you introduced anything new hardware or software wise?

If your temps are fine, turn the fan speed down and enjoy lower noise. Adding a second fan will likely add noise.

I know this is old but with that setup you should watch more than your cpu temp and see if you can reduce ambient temperature. also watch your vrm temps. as for another fan and noise, I would put another fan in front of your radiator you want low flow high pressure cfm push pull on the rad as intake on low rpm and a high flow low pressure low rpm as exhaust on the side. you can get tiny fan controllers that take up a pcie slot on your case, just plug it into a molex and your done.