People have had success with both methods so it's not an easy answer but I will share with you, how I do things and what has worked for me.
Here's the first rule I use when buying and importing discus fish; Match the water conditions of your supplier.
If you follow this rule you will be at a great advantage as the discus will already be accustomed to the water you provide.
This will cause a lot less stress than moving discus used to hard water into soft water.
Now your water doesn't have to match exactly but if it is about right things should be OK.
Don't go getting stressed if your hardness is out a couple of degrees.
Once fish are settled here's how I like to keep mine.
Young Discus Young discus fish should not be kept in very soft water as there aren't the right minerals present for good growth.
They should be raised in slightly hard water with a natural ph.
In this water they should grow quickly and healthy.
For my young discus I use tap water filtered through activated carbon and then left in storage barrels for at least 24 hours.
This will make sure all the chorine is out of the water.
Adult Discus Adult discus, once 5in should be kept in soft water but still not at breeding conditions as they may still be growing but don't need the minerals like young discus fish.
By lowering the softness and ph of the water you will trigger breeding activity between the adults and should get a few breeding.
For my adult discus I again use tap water filter through carbon and left to stand but I also add aquarium peat.
This softens the water a little and is fine because of the tap water I have.
You may have to use RO water mixed with filtered tap water.
Also if using peat to soften water, make sure it is of the aquarium variety.
Other garden peat can contain chemicals that can harm discus fish so just be careful.
On a final note, there is no right or wrong hardness to keep discus these days because they are tank bred through generations and have acclimatized to harder water.
Just follow these guidelines and you should be fine.