you can reach 4300-4660 attack on +11 trans 10 level 350 dagger (the one with the bonus damage on backstab) and 4681-5080 attack on +11 trans 10 equivalent sword.
The attack difference is really small imo. Also, while sword only boosts max patk with red gems, you’ll get a boost in both min and max patk with red gems on dagger. The only real benefit of sword is that yellow gems have more crit attack on mainhand than offhand.
judging from the description the additional damage % are flat and added on top, so it is 567+150 = 717%. That’s still a good boost, considering you’ll get 150x2x3 = 900% additional damage per CD (= sth along the lines of every 12 seconds).
let’s make an example:
lets say patk for dagger is 4500 and 4800 for sword, the backstab damage would be
34,02 x 4800 = 163296 for sword
43,02 x 4500 = 193590 for dagger
the dagger has less overall costs (upgrade/transcendence) and only loses out if we calculate in some really high-leveled crit damage gems.
It’s basically the same as Rod+Dagger on Thauma vs Staff on Thauma, dagger is more efficient with less investment while Staff is a little better with more investment.
However, if the Rogue plans to autoattack,too,as filler, Dagger becomes better once again because it has a higher base Aspd.