Assassination is the easiest spec of them all, but Sublety is better.
Assassa has sustained damage, you're playing around with dots.
downside is that you energy starve a lot and rupture & garrote are the 2 ways to regain energy. That's great, but bad thing about this is how almost every CC removes the dots (glyph of Polymorph, glyph of Blind, Glyph of Paralysis, Glyph of freezing trap (or w/e its called) and so on!)
Sublety has more controlled burst, you can peel your teammates and better synergy with different classes.
mage is a perfect example as dance is on a 1 min cd & frozen orb is 1 min. perfect synergy. :)