These two fat babies are currently contending for the exalted position under my desk (in an unwindowed case to boot, not like I wouldn't be anti-social enough to show off the Noctua beige).
I've seen a bunch of reviews and tests, starting from
TechPowerUp, and the results are somewhat inconclusive. Perhaps RT wins on Intel and D15 on AMD? I truly don't know. Sometimes the one, sometimes the other leads by one or two degrees or decibels. While temperatures are closer to evenly split, RT usually comes off quieter.
Most people would probably call it even and decide on the basis of the price. Well, let's just say that wherever I can realistically buy from, the RT is priced on par with the single-fan Noctua, the D-15S.
What I like about the RT is more RAM clearance — both my mobos (main and backup) have problems in this regard, even without tall sticks, and I think I've already broken pins in CPU slots due to applying some physical force to help things fit, so I need to be careful. (Would probably be buying the D15S with an additional fan instead, to help compatibility, though that's taking a slight performance hit due to the asymmetric build.)
My top 1 priority is silence, as I'm a bit sensitive to sound and find it difficult to concentrate on my work (reading and writing, often at night with almost zero ambient) if I can hear anything from the PC. Almost anything. Likewise, during moments of silence in a game, I don't really like to hear fan brushes, let alone rotors.
Thus, a single large fan at 400-ish rpm while not doing anything overly CPU-intensive is gold, as is that particular fan's (TY-147B) relatively low max.
However, I've also heard that while Noctua already beats most other coolers on pure decibels, it also has a comparatively less annoying sound, which may be important for when you do hear it. I think it's safe to assume the RT would be more annoying with the same decibels as the D15.
I've looked at AIOs, but I'm pretty sure I'll be able to hear the pump if I concentrate, so that's out of the question until I have enough time and money to build a decent custom loop.
I can't really think of better air coolers, though there is supposed to be a comparably priced air Phanteks (C14PE?) that's marginally better than these two, at least according to one large test I've seen somewhere. I don't know how it does that, looking almost exactly like my Gelid Phantom, with which I'm not quite happy, but anyway.
Finally, the TechPowerUp test, which is so favourable to the RT, does show the Noctua take back the lead by a single degree in the most extreme load + OC they did. Meaning the two fans are probably doing a better job at dissipation. This could perhaps become a bigger difference during a long gaming session @ all ultra with an OC'd CPU. But then, one can also buy another fan for the RT.
Does anyone own both of these coolers? (Or an AIO with a 100% inaudible pump?)