It is ALWAYS preferable to backstab. The damage increase is substantial.
My Scoundrel could usually outright kill one enemy on his first turn using backstab; Backlash over them (backstab attack), then Throwing Knife from behind for another backstab (best damage skill I've seen, seems to completely destroy all phys armor, 2/3rd's of magic armor and does 70% of their health damage too), then Flesh Sacrifice and a regular backstab.
Once I got the talent "The Pawn", which is 1AP free movement per turn, it makes setting up backstabs even easier if things like Backlash are on CD.