Sorry to say this dude... you can't dual to a "kit" ie. you can become a thief after being a berserker but not an 'assassin'...sorry
but :
i think that assassin is better if you like playing as a sneaky thief...you will have to sacrifice some skills but your backstab can go up to 7x damage at lvl 23 as well as the poison weapon...so, decide!
Too bad assassins can't have two points in weapons like swashbucklers...and have a crappy stronghold but hey... still good!
------------------
Yes indeed...