It depends on many factors, like cask principle. You have to find the shortest one, and calculate the marginal benefit of each one. Use the class who mainly use storm skills for example:
You have 45% storm dmg.
You have 90% dmg to x.
Your 80% of dmg is come from storm skills.
Marginal benefit of 30% storm mount: (30%/(1+45%)) *80% =16.6%
Marginal benefit of 30% dmg to x: 30% /(1+90%)=15.8%
According to this example, we know which is better mainly depends on this several factors: 1. How much elemental dmg u have? 2.How much dmg to x u have? 3. How much elemental dmg/total dmg u have? We cannot say, which is better, which is not directly. And the 3rd one will change. Like Hs, kill target by only one smite, holy dmg/total=100%.
To be simply (not very accurate, but in most cases) :
if Skill dmg / dmg to x >0.6: use dmg to x.
else : use elemental skill dmg.
We assume 0/0=1
PVP is a complicated one. I will try to say simply, and only talk about 90 pvp version.
In 90 pvp, normally, players give up -crit dmg. Because 90 gears give too much crit dmg. If you want to stack -critdmg, you will lose too much dmg.
So, If you want to be offense, Skills dmg, or almighty, depends on your class. like hs, lightning dmg. Katar, almighty.
There are 3 types mount is defense: Almighty, -x 30% dmg, -10% dmg.
Gears can be switched before it start. So you can see the name list to decide you use which one.
-10% dmg is a powerful one, when you don't have 75% dmg reduction. Don't say you can have more than 75% when the Heart and Stone. When you just came out, a +30 HS send you a Lightning, tell me where is your Heart and Stone. But there are many way to stack it, not necessarily to use mount. Like potion 10%+pvp potion5% +eido 15% =30% . Eido and pet can be switched during pvp. So, if you have solutions of dmg reduction, forget this mount.
-x 30% dmg. Use which one, not depends on which dmg you receive most. It depends on which skills is deadly. They are not quite equally. You may receive much fire dmg from Rava, but u can heal. And, who kill you is always a lightning hs.
The formula of -x dmg is equal to reduce others' skills dmg (if ignore other players). E.g. A lightning hs is using 30% lightning dmg, and you are using -30% lighting dmg. It is equal to both of you have no mount.
Almighty is simple. It gives you a lot of stats, both dmg and tank. But not that tank, and not to much dmg.