Outside the case means much more direct noise...I'd get decoupling mount on the radiator (usually for pumps) and dampening shroud for fans (especially if going for 2000+rpm...reminiscent of jet engine for my 2150rpm gentle typhoons on 30fpi rad)
As for quietness, Noctua A12X25 has really great performance per decibel (they also sell 120mm to 140mm adapter).
It bumps the silence rpm range from 700 of other fans to 1300, tolerable until 1600, then just fan noise @2000, no annoying vibration/resonance. It's on XSPC RX rad (thick, low fpi) though, so thinner denser fins may amplify the noise.
If you're going for push/pull, I highly recommend HWLabs Nemesis 140 GTX. It's rated to be best push/pull 140mm rad here: