Yes a 140mm is usually always quieter then a 120mm.
That being said i don't trust most db ratings manufacturers spec.
For instance i have a CM R4 Sickleflow 120mm listed at 19db lol! in your dreams maybe.
It's a decent fan though but i would guess closer to 30db at full speed which is pretty good actually.
On the other hand i have Thermalright fans and they are rated correctly + or - a couple of db.
Same goes for Scythe fans.
You get what you pay for.