Though they kinda share the crown of The Chase and Escape King, they are vastly different gameplay-wise. Shaco is best used as jungler, while his highlights are made of cunning mindgames, strategic JitBing, bold and daring swashbuckling in the nick of health and other adrenaline-filled nonsense. Kassadin, on the other hand, is like steamroll on meth: he gloriously enters the fray in the pool of azure pain, nullball falls, everyone dies. He's all about brute disables and crapload of semi-ranged burst, little to no finesse here. Choice is yours. But overall, I'd say, binary choices are better solved with taking BOTH options. Shaco or Kass? Why, of course Shaco AND Kass is better. IPs are trivial matter, really.