Endurance Potion

Ironskin hands you defense, which comes off every hit by the same amount. This gives you a percentage cut instead. Against weak enemies chipping at you the flat defense is worth more. Against anything that hits hard, and that means every boss worth drinking a potion for, the percentage wins by a wide margin.

They are not alternatives. The two effects apply on top of each other and neither one blocks the other, so a proper boss loadout carries both. The same is true of the damage reduction on armor and accessories, which is how a heavily defensive Hardmode setup ends up taking a small fraction of what it should.

Burning goes straight through it. On Fire, Frostburn and Cursed Inferno drain you at full rate regardless of how much reduction you have piled up, so the answer to those is still getting out of them. It is also worth knowing that your buff bar has a hard ceiling, and a full potion spread plus a couple of debuffs during a long fight can quietly shove something off the end.