Lifesteal vs damage buff vs hp buff
Let's say you can deal 100 damage before you or you enemy die, let's compare how much more damage you can do with 10% lifesteal and 10% damage buff and 10% hp buff.
For 10% hp and damage buff assuming your DPS is the same all the time, the extra 10% hp will translate to 10% more time to DPS, so 10% more damage so 110 damage. And 10% damage buff is just 10% more damage so 110 damage.
For 10% lifesteal things get complicated, you get 10% of that 100 damage as your HP so 10 more HP for you. If your HP is more than 100, you will deal less than 10% more damage so less than 110 damage, and if you HP is less than 100, you will deal more than 10% more damage so more than 110 damage.
You are in a weird spot where you are rewarded more the less HP you have.
You can artificially create this advantage by using soilder armor to artificially lower your HP, talking about the HP you are effectively DPSing .