Based on a review that I found yesterday the XSPC RX or Alphacool full copper rads are the best.
This comparison is based on 360mm rads but the performance should still be the same for the 420s only difference being lower temps and a slightly lower flow rate due to them being longer.
After all they are the same RADs with the same FPI count and internals, just longer in length than the 360 RADs.
There is a difference in performance, you will get lower temps at lower fan speeds with a larger rad.
Going off that comparison of the 360s, if you were to change them for 420s, where the different models are on the list they should stay the same. So XSPC RX would still be top and Alphacool second etc etc.
The only difference would be the temps would be lower than the 360s and flow rate would be lower because of the extra length. The different rads would still be in the same position on the graph as the 360s though.
If you see what I mean.
For fans you want to get the fans with the highest static/air pressure (mm H2O) and the highest CFM you can find but with the lowest DB you can find. A high static/air pressure is more important than a high CFM but you still want to get fans with the highest CFM you can find without being ridiculously loud.
You will have to do a bit of googling to find out the static pressure of some fans though because it's not always listed in the specs.
With the longer thicker rads that have a low FPI though you want fans that work well at low RPMs which is what those type of rads are designed for which is also why a higher static/air pressure is more important than a high CFM. So make sure the fans you choose also work well at a low RPM.
Its not the best choice for 420 rad on specialtech, so I think I go for Coolgate, or Phobya one but I think that I'm closer to Phobya, because is 26pounds cheaper and perform pretty much the same(half a degree difference is not a big deal for me
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There is no really good 140mm fans with good SP and silent, NZXT one have good SP at full speed but they are not the quietest So I go for Scythe Gentle Typhoon 1850rpm and Alphacool NexXxoS UT60 360. Cuz this fans are very quiet and perform very well, also a lot of people recommending them for the rads. They doesn't look pretty but, it doesn't matter when they perform that well
I have a coolgate triple 420 radiator here with 6 spectre 140mm fans running push and pull, the fans are extremly smooth running and quiet even at full rpm. There have been a few videos put up of these fans making clicking noises but thats all tosh.