You are not talking about Damage Mitigation.

You are talking about the differences between Helm Smasher vs. Flawless Executioner.

Helm Smasher is considered to be hands down better vs. Flawless Executioner.

Helm Smasher ignores 25% of the enemies Defense.

It simply allows you to do more damage vs. Flawless Executioner in the majority of cases.

However, people still use Flawless Executioner!

Flawless Executioner (+20 C. Damage) is still regarded as a very good Mastery despite being "Weaker."

The reason why is because Flawless Executioner is very helpful when you want Consistent Damage.

Helm Smasher is "Better" only when it Procs which is 50% of the time.

Most players don't like using Helm Smasher in PVE related content because of the 50% chance variable.

Most players like making "Farm" teams or Campaign "Farmers".

They want these Heroes to have a 100% win rate.

They want to auto-run hundreds of thousands of battles!

It can be risky running Helm Smasher in area's of the game where you want Consistency!

This is why people I know like to run Helm Smasher on PVP related heroes.

Can players use Helm Smasher in PVE or PVP related stuff? Yes!

Can players use Flawless Executioner in PVP or PVE related stuff? Yes!

Players can use these Masteries any where they want too.

The main take away from my post is:

Use the T6 Mastery which best fits what you are trying to accomplish!

Helm Smasher = does more damage, but is risky damage (50% Chance)

Flawless Executioner = does less damage, but is consistent damage (100% Chance)

I am talking about damage mitigation as the defense = damage mitigation, and my entire post is about the amount of damage added from removed defense, or the amount of damage not being mitigated by helmsmasher (as 25% armor reduction =/= 25% additional damage). But yes, it is also about how this difference results in the difference of damage between helmsmasher and flawless executioner (which would not fit in the description title).

The closest approximate formula for DR we have, based on some user testings is something like this:

DR = DEF / (600 + DEF)

Now, i am very bad at maths (almost bad as with English grammar), but it seems like it wouldn't be easy answer to just call HS or FE better. It will be correct answer in some cases and not in others. Seems like the more DEF your target have, the more value you will get from those ignored 25%. Also, seems to me that FE start to "weight" less when you start to stack some serious CD numbers.

To call one or another "better" without consider the primary use of the champion, probably his stats (and even the other masteries which is required to obtain those) is somehow half the answer.

Also, I think it is almost impossible to correctly measure damage boost from those two for the simple fact that you can't have exactly the same mastery build for FE and HS. You have to sacrifice things like Kill Streak and/or Methodical and they still contribute to your damage.

Most of the time i must consider other masteries as well, and the champion himself, and the content i mostly use him and just after that i choose FE or HS. Seems for me that as so called "end game player", i choose most of the time Helmsmasher, as most of the time i want those Methodical and Kill Streak and +20% CD isn't that critical for me. It is not that Helmsmasher is better, it is better for me most of the time, considering all other variables i mentioned.

Also, seems like some of those content creators that i consider really endgame players and with way deeper understanding of the game than all of us combine prefer Helmsmasher over Flawless Execution 9/10 times.

*Edit - @L9753

I was bored so I ran some experiments, but I see Daering already shared the same conclusions as mine (for once you're faster than me ;) ). 

I did a few battles with Elhain and Stag Knight, testing her Lightning Arrow with and without his decrease def debuff - of course his defense reduction is 60%, it might have been clearer with a champion doing 30% reduction, but I didn't have one readily available. 

For a fairly high defense opponent (Kaerok Castle boss on Nightmare), she did about 25% more damage with the 60% decrease def buff on, than without it. With a skill ignoring/reducing defense by only 25%, I guess the actual damage difference would have been much less, perhaps somewhere around 10%, although it's probably not actually linear. 

For a low defense opponent (Clan Boss on Normal), she gained less than 20% of additional damage from SK's debuff - because the low defense was already stopping only a small part of her maximum potential damage output, so reducing that defense further didn't make as much difference anymore. 

On the other hand, with that 20% critical damage bonus, your actual damage increase also depends on what critical damage you already have, as Daering also mentioned. If you currently have the minimum of 50% crit damage, then FE makes it 70%. So your crits now hit for 170% instead of 150% of normal damage, which is a 13.33% damage gain. But if you already have 150% crit damage and FE makes it 170% (so crits hitting for 270% instead of 250%), that's only an 8% damage gain.