ADC Starting items: Long sword+3 pot or Doran's+1 pot?
At early laning phase, additional health and potion are essentially the same. So is lifesteal. So which is worth more: Doran's Blade start, or Long Sword start?
Common defense for Doran's Blade is its high gold efficiency, but you have to realize that potions are extremely gold efficient (35 gold for 400 gold worth of health), at least in laning phase. Each health potion restores 150 health total, if you don't take utility masteries. This is as much as a ruby crystal. Doran's blade give you 70 health and 3% life steal. The life steal shouldn't be calculated as gold efficiency of the stat, but rather how much health it can grant over the course of laning phase. We'll get to that later.
Just from potions and extra health, here's how much health you get from each start.
+ 3
: 450
+ 1
: 220
So in order for Doran's start to be worth more, life steal will have to grant more than 330 health. Since it's granting 3% life steal, you have to do total of 11000 damage from AAs. It's difficult to calculate how much that is, so I'll guesstimate. Assuming average of 100 AD, you'd have to autoattack 110 times. That's slightly more than the number of minions (in 10 minutes), and since you'll probably fight each other at some point during laning phase, that's enough damage for lifesteal to be worth more than 3 potions. But it's important to note that potions can give you health anytime you want, whereas lifesteal gives you health slowly. Having 150 more health when you need it rather than later can save your life.
Another argument is that Doran's Blade+IE will grant decent sustain. Well, so would Vampiric Scepter+IE, and Vampiric Scepter builds out of Long Sword and 440 gold. And you don't have to sell your Vampiric Scepter since all the lifesteal items build out of Vampiric Scepter.
Final point I want to make is the support. If you have a healing support, you don't need the early lifesteal, since at early point of the game lifesteal just serves as a form of slow sustain. With a healing support, it's best to use Long Sword+3 pot start because you don't have to sell Doran's until much later.
I probably overlooked a bunch of things (stacking Doran's Blades comes to mind), but I personally prefer Long Sword+3 pot start for the extra AD and not having to sell Doran's blade. What do you get, and why?