<Stolen from the rogueforums, i'm no numberwizz>

Thought I'd do the maths behind this to show why Felsteel Longblade is considered a bad choice for Rogues.

I am not factoring in misses, parries, dodges etc as it's not needed to show the difference between weapons, unless a particular weapon has a +HIT stat on it, which neither of the swords in the example given do.

Firstly, white damage:

Duration = 1minute.

Felsteel Longblade
27 hits, each doing an average of 185.36 damage (128 - 239 damage range + 26AP from the AGI)
Total damage = 5004.64

Edge of the Cosmos
23 hits, each doing an average of 188.64 damage (130 - 243 damage range + 30AP)
Total damage = 4338.79

So on base white damage, Felsteel Longblade is a lot better.

Sinister Strikes
I'll assume full points in Lethality, Aggression and Surprise Attacks.
So the formula would be as follows:

[([(Attack Power / 14) * Normalized Weapon Speed + Weapon Damage + 98] * 1.06) * 1.1] * 2.3 = Damage

Normalized Weapon Speed = 2.4 as SS is an instant attack.

I'll assume a base AP of 1500.

If you only SS during the course of a fight and your timing is perfect, then you can expect to hit 18 SS's in one minute (20 energy regen per 2 secs, not including any talents/potions etc that will regen energy for you)

So SS damage as follows:

Felsteel Longblade
18 SS hits @ 1444.53 average damage per hit = 26001.54

Edge of the Cosmos
18 SS hits @ 1452.58 average damage per hit = 26146.44

Total damage for both swords:

Felsteel Longblade
26001.54 + 5004.64 = 31006.18

Edge of the Cosmos
26146.44 + 4338.79 = 30485.23


So given perfect circumstances, Felsteel Longblade will just about out damage Edge of the Cosmos.
But considering the cost/time involved in getting each one, Edge of the Cosmos is a clear favourite.