While max health is based on how much health a champion can have current health is based on how much health the champion has at the time. Because of this current health damage is less effective against champions at low health than when they're at high health while max health damage does the same amount of damage against an enemy at any health. Say, for example, I'm using a Mundo max level Cleaver, an ability that does damage based on a champion's current health, on an enemy at 500/1000 health. This Mundo Cleaver will do 25% of the current health of the champion, meaning a quarter of the 500 hp that the champion has at the time. This will be 125 damage. However, if that same Cleaver hit the champion when the champion had 1000/1000 health it would deal 25% of that current health, or 250 damage. Meanwhile Vayne's level 4 Silver Bolts, an ability that deals damage based on the enemy's maximum health, will do 10% of the player's maximum health. Let's take the enemy with 500/1000 health again, even at 500 health the silver bolts will do 100 damage because 100 is 10% of 1000. No matter how much or how little health the target has remaining Vayne's Silver Bolts will do 100 damage on an opponent with a max health of 1000. Hope that answers your question.
I usually rush Frozen Heart against an AD opponent regardless if I want to do AD Sion or tank sion because it's a great item on him. Fixes mana issues and gives tons of CDR, armor, and an amazing aura Also while a lot of lifesteal items aren't good on Sion I find that a Hydra is reasonable due to its passive and active allowing him to CS better.