Personally, both CK and an IP are essential in a dagger assassin arsenal. IP = more damage the more defense the oponent has, perfect for bulky types like those from the Swordman school. Combat Knife, not only gives you a bit of protection, but also pierces through the oponent's defence, Generally used to fight easy things like those from mage school or acolyte school.
If you use an IP vs someone with poor defence, your damage will be pathetic lol.
I would get IP and CK and save the cash from making a SST.