User blog comment:Epegi/Hp/5 Vs, Lifesteal in the Early-Mid Game/@comment-3067750-20110929120027

HP/5 is the entire reason champions like Heimerdinger, and to a lesser extent, Garen and Mundo, are very resilient to early game pokes / harassment that is drawn out over periods of time. They each have enhanced HP Regen compared to others by their design.

As with % vs Flat Armor Penetration or % vs Flat Magic Resist Penetration, and especially % Max HP Damage (Bloodrazors) vs Flat Damage, you must weigh the benefits of the % value vs the flat value.

Against Low Armor / MR targets, Flat Armor / MR Penetration will be more profitable while % Armor / MR Penetration is more profitable vs high Armor / MR targets

Against Low HP targets, Flat Damage will do more while % Max HP Damage will do more against High HP targets

The same is true here, with Low Damage, flat HP Regen will be better but with High Damage, % Lifesteal will heal for more.