So, people have argued over this on Gamefaqs before.. but I haven't gotten around to putting up the math behind the argument of which is better.
So, let's go over the basics.
Deadly is 3x attack power.
Double Jump is dealing 2 Jumps.. this isn't doing 4x damage mathematically due to defense factored into the calculation.
So, anyways, which command always does 9999 damage under optimal+worse conditions? Let's find out...
If the monster has 254 defense...
For Deadly...
224 is used for the sake of argument, given that Yang's attack power is limited to that... numbers will be rounded up
9999/(224*3 - 254) = ~24 multiplier is required for 9999 damage
9999/(255*3 - 254) = ~20 multiplier is required for 9999 damage
For Double Jump...
9999/(255*2 - 254) = ~40/2 = ~20 multiplier is required for 9999 damage
Double Jump is only slightly stronger overall if attack power was maxed (at 255)... however.. Deadly will never have such an attack power to work with (unless hacked).
However, people forget the details of Deadly and Double Jump when making these comparisons...
Deadly/Power:
All magic evasion drops while using this command. This means that Yang is a magic damage magnetic, especially when he has the worst magic defense of all existing characters.
Jump:
The character is in Jump status where it cannot be targeted. The only real negative is that spells that do split damage will hurt more.
So, Double Jump > Deadly. Period, end of story.