This is very interesting!
Where did you get the info that the C14s is better than the U9s?
Do you think even in a dual fan config for the U9s?
I am trying to figure out which is better for cooling a 9900K with no power limits.
In the C14s case, the rear fan will be in exhaust.
With the U9s maybe I will try to have the fan placed as intake from the rear and the side fan as exhaust actually...
If you have more info you save me from buying both!
A stock U9S should be compared to a stock C14S. If U9S is double-fanned, then C14S can also be double-fanned too.
The general data seems to suggest that C14S has better thermal performance. C14S is 820g, 6 heatpipes and stock fan moves 140 m3/min max vs U9S' 524g + 5 heatpipes + 80 m3/min max.
Example reviews saying C14S > U9S:-
http://www.relaxedtech.com/reviews/noctua/nh-u9s/4
https://www.guru3d.com/articles_pages/noctua_nh_c14s_low_profile_cpu_cooler_review,8.html
https://www.tweaktown.com/reviews/7284/noctua-nh-c14s-style-cpu-cooler-review/index6.html
But then, there are some reviews saying U9S > C14S:-
https://www.overclock3d.net/reviews/cases_cooling/cpu_air_cooler_mega_test/39
Additionally, Noctua itself says U9S can handle 215W but the C14S can handle only 190W. Both coolers are rated as ''compatible with certain restrictions''.
https://noctua.at/en/cpu/Intel_Core_i9-9900K
I think, for most situations, C14S will perform thermally better. But in some cases with some airflow arrangement, U9S will be better. However, if you wish to truly max out an i9 9900K, perhaps neither are very suitable. But, if you have to choose between the two, then you will have to actually buy both and try them inside a DA2.
Since DA2's general internal layout is similar to M1, I suggest you might go through the M1 thread to look for hint. I did that myself a few months ago as there are many people using C14/C14S/U9S inside a M1. My general feeling and memory is that, if the airflow is managed well, C14/C14S edges out U9S slightly thermally.
Of course, there are other factors to consider. Eg C14S is so huge that it blocks your access to the motherboard. It might have memory height limitation. But, it blows air down onto the motherboard and cools it. This factor may (or may not) become important, especially when the motherboard is inside a small (SFF) case where the internal is usually densely packed, thus limiting the establishment of clear airflow path(s). The smaller number of case fans of a SFF case does not help either.
Anyway, whether you try both or either one of them, I look forward to read your result.