Of course IE is a lot better because it costs 1300 Gold more.
However it doesn't stack well on it's own because it only get's you 30% extra dps until you buy more crit items.
Let's say for the sake of math Ashe has 80 AD and 1 AS (that's roughly level 12 with Berserkers)
Then she get's IE:
(160 AD * 1AS) * 140% crit = 224 DPS
alternatively she gets a SS and a BFS (slightly more gold than IE)
(130 AD * 1.5 AS) * 130% crit = 253,5 DPS
after having spend almost the same gold we are already at more damage, without the SS passive. But we also have an easier build path and thus a better early game.
As far as first items go IE is NOT the best choice because it increases crit and you don't have much crit chance.
Though I'm not sure how well the extra early AD would interact with Flurry and Volley.