There is also another dagger that is 14/28 that is usually cheaper than Rez by far. Can get it for 200-250p and it is 14/28 +20 HP. Not as good as 20 dex but far more affordable.
Another thing to consider is the ratio, due to the good ratio, the usable level on these is higher than some would like.
Stats with Backstab, I don't know really. I recently upped my rogue (21) strength from 125ish to 150ish and saw no real difference in his BS amount, but I also do not parse it.
Sorry that I could not be more help.