User blog comment:Kharjou/ADC tips chosing first big item/@comment-5514182-20130408233403/@comment-7923992-20130409111844

if you watch closely  a  Q with BT roughly deals 45 more than with IE

and since AA deals 15 more damage here  with IE than BT  here

you need 4 attacks to actually deal more with Q + AA pokes  with an IE, and on IE you get no 18% lifesteal, which is huge.

if you poke   with Q then manage to attack 4 times either you have a red buff or  the other carry is picking the fight,   and as cait you should only do one Q + 1 or maybe 2 AA in lane to   abuse her range without getting hit

so for poke in lane and sustain BT is shown to be much better.