Is flat damage reduction better than % in pvm?
Ive heard somethings but im unsure

Both are good to have.

Integer DR comes before %DR though.
So if you get hit for 100 dmg and your only source of %DR is a ber'd shako, you'll take 100 - 18% = 82
If you have a sol'd shako you'll take 100 - 7 - 10% = 84

You notice the effects of int DR more on low damage attacks and you'll be happy to have %DR in situations when you're amp'd or against extra strong fanta packs.

this:

is precisely why this:

is wrong

let's say you take 100dmg while amped:
with 10%dr you lose 100dmg*(100%+100%amp-10%dr)=190life
with 10dr, you lose (100dmg-10dr)*(100%+100%amp)=180life

So flat reduction is better for pvm?

for general pvm, 1dr is about as good as 1%dr
if you wanna make an immortal char, dr% is useless

Interesting so one should sol his gear rather than ber in pvm?

If i may ask if you have both flat and % will they have dimishing returns?

i wouldn't call that diminishing returns, it's just that they don't work well together because they're applied in the "wrong" order.
you want to reduce in percentage first, and then by an integer
example with 50%reduction and 50integer reduction:
The "right" order: 100dmg -50% = 50dmg; 50dmg -50 = 0dmg. <- this is how abs% and integer abs work
The "wrong" order: 100dmg -50dmg = 50dmg; 50dmg -50% = 25dmg <- this is how dr% and integer dr work

if you wanna be immortal, you stack integer dr, and then no dmg goes through your integer dr so there's no point having 50%dr to divide nothing by 2

In general no, this comes down to build effectiveness

Most builds can get 30% dr or more without sacrificing kill and movement speed, some can hit 50%, this is all you need really

With ipdr you usually have to go seeking it out, which is not worth it in almost all occasions

Integer reduction is probably quite useless except in specific situations, such as immortal characters or eshield sorc. Otherwise, you will notice very little difference from integer reduction. With 15-50% damage reduction you should feel considerably more tanky. And if you add max block into the mix and some ele sorb you should be effectively immortal despite still taking some damage

Yes. Flat damage reduction can yield most monsters to do no damage given enough DR. That's why Gladiator Bane has such a high level. It was intended by the developers to be useful in the game PvE wise. 20 DR and 20 MDR is a lot in PvM. A monster may do 50-60 damage but have 3k HP. When you add more players to the world the monster HP rises but the damage is the same. You can easily achieve over 100 DR with max block and block speed on a paladin and become god-mode.