I want to be an assassin

If you're looking to maximize your effectiveness, you'll want to hit 20 Strength (for gear), 30-36 Dexterity (depending on talents), and dump as much as you can after that into Cunning.

Cunning has several benefits. Cunning increases your armor penetration values, and the Lethality bonus allows you to derive your Strength bonus from Cunning instead. Furthermore, the Assassin's Exploit Weakness talent adds an additional backstab damage modifier based on Cunning. These factors mean that once you have sufficient Strength and Dexterity, Cunning becomes the more valuable stat for DPS.

Finally, Cunning improves lock picking, which is a very useful talent. After reaching 30 Cunning, your next 10 points are equivalent to one free rank of the lock picking talent. Meaning at 40 Cunning you only need 3 points in lock picking to pick all the locks, and so on.

Dexterity will greatly benefit assassins in sheer damage output of daggers (unless you're some kind of weird sword-wielding assassin) as well as providing nice defense.

Cunning will help more with stealing/stealth etc. but that seems to be a different character archetype altogether. The armor penetration bonus gained by cunning will be overshadowed by the sheer damage boost from a dexterity-heavy build.

One kind of neutral point to make here is that Cunning will give you some mental resistance while Dexterity will give you physical resistance -- but you probably shouldn't make super crucial decisions based on that information.

There are no minimum values for either attribute to become an assassin you simply have to speak to the right person. However, for all combat skills a combat-focused rogue will desire, you'll need a minimum of 36 dexterity (dual-weapon mastery is the most restrictive). A cunning value is only needed for disabling traps, performing stealth, and skills -- none of which I feel is crucial for an assassin archetype.

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)