As you can see there is a good measurable difference between the two cooler. For both coolers I can not recommend passive cooling. Although the U12 is capable of it I do think the temperature is slightly too high for this processor. Granted non-overclocked the temperature would definitely be a few degrees lower but inside an actual case the ambient temperature is higher also... so no that's not an option for me personally.
A choice in coolers is definitely something you need to consider. Once cooled with a fan the overall difference between the NH-U9 and NH-U12 is very small. So you need to consider which model you find the most attractive. I'd still go for the U12 model personally.
Which fan would I opt for ? Definitely the low-noise fan. You can't even hear it so that's just fantastic cooling performance versus an inaudible environment. The high performance fan however when it's at full rotational speed can clearly be heard and when you look at the performance differential between the two fans there's a 2 maybe 3 degree difference overall, so the high performance fan isn't really making a significant difference. Perhaps if you want to overclock your processor to extensive heights is where the high-output fan will make the difference for sure.
Luckily you have options. I must say that I'm really pleased with the products from Noctua. For now I'll leave the U12 cooler with silent fan on our test system. It's actually working really well and that surely says something about the quality that is being offered here. Other then the sheer size of the product there's nothing negative to report here.
There is something else in favor of the U12 model as well, it's the price. Although these coolers are not exactly cheap at 40-45 EUR the difference between the U9 (45 EUR) and U12 (50 EUR) model is roughly 5 EUR. These are e-tail prices I found here in the Netherlands. So why would you not buy the U12 model then ? I mean unless you need a smaller size cooler to fit your PC the U12 obviously offers slightly better performance.
Oh last but not least, if you bought a Noctua U9 or U12 product in the past and decided to move onwards to socket AM2 then you have an issue. The cooler bracket's will not fit. Lucky for you Noctua will send you a Socket AM2 upgrade kit for free if you mail them. That's classy and fantastic after sales in my eyes.
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