I know there has been a lot of confusion with defense after the update, but this will tell you some things about defense.
First off, armor reduces damage % wise. The new update buffed defense so I think a new Meta is coming where STR heroes that are tanks are becoming the new Meta(characters that Increase physical damage with their STR). This was the Meta but only for a little bit.
Every Character has a hidden value called Defense. Defense Mods that increase defense %, increases the hidden value. Therefore, You won't see an exact armor % increase. 20% armor + 10% Defense from mods will sum up to 22%, But You won't see exactly 22% you would see something around that like 21.89% Because it we are adding defense, not Armor. The hidden value adds about .1 to .15 to the armor stat. The hidden value undergoes a formula that changed the value.
Then Mods sum up % into a total number, so 2 mods that increase 3.75% will have a total of 7.5% increase to armor. So if that character has 100 Defense (NOT armor) the new value will be 108 then for the extra 8 Defense points will increase about .08 to 1.4, Depending on the character. This is not always the case, increasing .08 to 1.4, use this site to help calculate defense.http://realsithofgenius.blogspot.com/2016/09/defense-and-armor-calculator-version-2.html?m=1
Is a health set better than a defense set,
Short answer, Yes. Here's the math, Let's say this character has 1000 health and 25% defense. We would want to know how much long they last. So this character against 100 damage hits
Decreased by Armor so each hit would deal 75 damage to character. Said character would survive 13.33 hits from attacker without any sets. The health set add a total of 15% health so the character now have 1150 health taking 75 damage per hit, he could survive 15.33 hits. But with extra 15% defense he would get about 28.75 Armor so now takes 71.25 hits at 1000 health surviving 14.05 hits. Therefore Health is better
This can vary between characters like characters with heal of defense up, or extra defense, but in general, health is Better
Sorry for jumping around a lot. A lot more coming soon Read the Gold notes for better Understanding