In terms of Attack Speed, the calculation is below:
Attack time is:
Base Attack Time / (1 + Attack Speed)
and your attack speed cannot go higher than 400.
Sniper has 21 + (2.9*level) agility and 12-18 base damage (lets call it as 15).
Considering that you are 18, you will have 74 agi, hence 74 AS
MoM gives 100 AS when active
Eye of Skadi gives 25 AS, 25 damage
Shadow Blade gives 30 AS, 22 damage
Manta gives 41 AS, 26 attack
You will have 1.7/(1+2.7) = 0.4594 attack time and 162 damage before you buy either items. You will also benefil from all of your headshots in that situation which is +36 increase to your damage. Your DPS is 431.
If you buy Butterfly, you will gain 30 AS and 60 damage.
That makes you have 0.425 attack time, and 258 damage: 607 DPS.
If you get Daedalus instead, you gain 81 damage and %20 chance to crit for 2.4 x your damage. Which roughly translates to %48 increase after all modifiers, except one fifth of your headshots won't proc. It still makes you have 873 DPS though.
In that case, Daedalus wins by a %44 advantage in DPS but if evasion is exactly what you need, you might need a butterfly. I also would recommend getting a Black King Bar before you even go for Eye of Skadi since sniper can be an easy target for nukers, and you will counter Blademail.