I am planning on building a silent but powerful PC with more emphasis on the former rather than on the latter. In other words, without sounding too fancy, it's important that it is quiet. So the system is going to be a cookie cutter 4770k and GTX 780 sli etc. I am also going to use the h100i with noctua nf-f12 fans. As a matter if fact, im using noctua fans wherever i can put them, replacing all stock fans.
However, i cant decide weather i am going to use the define r4 or the corsair 750D. Will the r4 be any hotter due to its isolation? Will the 750D be louder due to its more open design compared to the Define r4? and even if it is not, what are the advantages of the 750D over the define r4.
the foam on much of the define r4 wont effect thermals as much as you might think, as long as you have good airflow/ exausts then it should not matter. You dont need to replace the stock fans for the most part, just go into the bios and change the rpm settings. unless of course you want that cream and brown color scheme on all your fans(I own two nf-f12 fans btw). XD
The 750D is made for performance, not acoustics, so the Define R4 will definitely be quieter. The sound dampening on the R4 is some of the best available. The 750D on the other hand will produce cooler temps, especially with an SLI setup. You will get more airflow in to the graphics cards because of the large intakes at the front of the 750D. The 750D will also exhaust air better, It has a much more free flowing design than the R4. The 750D is also made with the corsair H100i in mind, the spacing from the top of the motherboard to the top of the case, is perfect for the H100i.
Acoustics in mind, the R4 is going to be very quiet. With all the sound dampening foam it is super quiet. Especially with the noctua fans. That's not to say you can't get the 750D to be quiet. If you used the noctua fans everywhere and used a good graphics card cooler like the Asus DC2 or Evga Acx cooler, it should be whisper quiet.
Conclusions. The R4 will be quieter with identical parts vs. in a 750D. The R4 is made to be quiet. The 750D is made to perform better in temperatures. The 750D may be 3-4c lower in temps, because you can get hot air away from the graphics cards and cpu cooler, whilst throwing massive amounts of cool air into its place. That's not to say the 750D can't be quiet with better part selections, its just that the R4 will be quieter but whilst running slightly hotter in the process.
Basically, R4 for quieter PC, 750D for better cooling.
I honestly prefer the aesthetics of the R4. I'm not a fan of huge boring boxes (I own a Lenovo and an NZXT Phantom, both of which have plenty of lines going everywhere). It's just that due to the existence of the 350D, everything else like 650D, 750D, 900D just look like really stretched out versions of it and it's quite weird.