I've seen people start with 
, others with 
, but which one is better to start with in general?
I know there are some 
exceptions, but for other ADCs, you have to choose between the two. So, which is better early?
Bt defensive ie offensive
Just got a double kill/first blood and want to snowball? Get that RNG bullcrap that critical strikes every other AA even when you only have 20% crit chance. on the other side of the above scenario? Get that bloodthirsty blade so you can sustain in lane when the enemy jinx crits you twice for 3/4 your hp.
Okay, so this comes up a good amount, and it's very important: DO NOT GET IE FIRST UNLESS YOU'RE SUPER AHEAD. the reason is simple: its value is completely based on your crit chance. which is 0 be default. so (ignoring the AD, because it's the same as bloodthirster) what your gaining is: a 25% chance to do 50% more damage than a normal critical strike. This really isn't very much. However........... 
, This item has the AS you want. it has more crit than the IE. it has a very nice passive AND: when you get your IE next or whenever, that 25% chance for a 50% extra becomes 55% chance. PD (or static shiv if you're into that) will make your IE much stronger when you get it, and will give you better stats for that early power, since you really can't utilize an IE until you have more crit.
as for BT, if you're doing no damage, whats the point of lifesteal? grab the PD or something, BT just doesn't compare unless you've got insane bass power like a draven or such.
Ima be honest here, BT is a very, very late-game item. Infinity edge will give you more dps early-game (sustain without good AS or AD is useless if the enemy bursts you out).
If you want early game sustain, a good item is Blade of the Ruined King. This gives good sustain, good attack speed, decent AD, and a killer passive that ensures it stays relevant. The active is a decent peeling/chasing tool as well.
I choose BT for a few reason
IE does an extra 300 damage every 5 autos. BT heals 100 damage every 5 autos and has a ~180 shield at lvl 9. plus if you trade in lane you can just farm a wave under the turret and get back to full health and trade again until you dwindle the IE user down. Also IE builds are very weak versus early thornmails if you ever jungle.
