For a melee rogue/assassin, cunning is straight up better than dexterity after meeting requirements.
For dual daggers (with lethality) each point of cunning provides the same damage bonus as dexterity: Daggers get 50%dex/50%cun(str) to damage. For sword+dagger or dual sword, you get 100%cun(str) to damage. Only Archery doesn't get a direct damage bonus from cunning. Add the bonus to armor penetration on top. Dex improves chance to hit, but even with max difficulty you will always hit with dex 36 due to flanking bonus (+20 which is requirement for backstab, anyway).
But what really makes cunning far better than dexterity is its influence on talents. Exploit Weakness (Assassin 2) adds bonus damage based on cunning, if you go rogue/assassin/bard, song of courage (bard 3) adds cunning-based bonus to attack/damage/crit chance. Also, some talents use cunning for success chance (your cunning vs. enemy physical resistance).
Non-combat, cunning influences persuasion, stealing and lockpicking.
So, unless you want to be a ranged rogue, i would recommend the following stats:
- 32 dex (36 dex with fade boni, req. for lvl 4 talents)
- 16 str (20 str with fade boni, req. for best light armor)
- XX cun (the rest of the available points)