Why oh why a katana and not a rapier?
Rogues can get rapier proficiency for free and it is a light weapon for most purposes (so you could use weapon finesse to improve your to hit rolls and use it with an off hand light weapon for even more sneak attack damage).
A katana requires exotic weapon proficiency (katana) to use 1 handed otherwise it is a 2 handed martial weapon and neither of your classes get martial weapon proficiency so you are at -4 to hit using 2 hands with no Str mod to increase for the 1 1/2 times damage.