Assassination is way more complex than Combat. Let's compare the two:
Combat
spam SS
use eviscerate to dump cp's
maintain slice and dice
maintain RS
use AR and KS as you see fit
Assassination:
spam mutilate
use envenon to dump cp's
maintain rupture
maintain envenom
use vendetta as you see fit
Ummmm............../scratches head.
/irony mode off
yes, that was a bit over simplified but i hope you get the point.
On a slightly serious note, I prefer assassination simply because getting two or three cp's per mutilate is just more satisfying than one or two per SS.
There is, however, one issue that, while admittedly not commonly occurring these days, does require an assassination rogue to think on his feet. When fighting in front of a boss (again, not common but it happens sometimes), an envenom can be dodged. When this happens, 80% of your energy is refunded, you keep your cp's, and the envenom debuff is still applied to the boss. So there you are, with 5 cp's, probably a lot of energy since you had probably pooled, and a full debuff on the boss. What are your choices.
Envenom again. Rarely a good idea since you'll be clipping your previous envenom badly.
Rupture. Usually your best choice, unless your rupture has over 6 seconds remaining.
Let the debuff run out and continue to pool.
None of these choices is automatically the best one, and you usually don't have much time to decide.
Again, not a common occurence, but sometimes you find youself in the wrong position and it happens. Just mention this to point out that the spec actually has an element of complexity that the other specs don't have to deal with.