Warrior or Elementalist

Playing a "sin wannabee on rit armor" is much more fun than playing an actual sin. Also, you have 75AL from Ghosteforge insignias. nub. The Rt/A is a gimmick and whilst it may do a lot of damage in packets, ultimately the energy management is better on the sin, you can take critical strikes to increase critical hit rate, you can spec higher into dagger mastery and it leaves your secondary open for support skills.

If you go Rt/A you are limiting yourself to using Spawning Power which (whilst Spirits Strength is strong) doesn't have any inherant bonus which would aid you to kill, which is why it is outstripped by Critical Strikes (as it serves both as energy management, and to increase your overall damage through crits).

Assassin's kill very effectively with little set up, maybe one enchantment (often not) and then they can just go at it. With a SS Rit you are typically renewing your skills every 20 seconds or so (Spirits Strength, Weapon Spell, Sight Beyond Sight) this will slow down your damage and leaves less room on your bar for an IMS to pursue the enemy which would have kited away by then.

Finally, the base damage on daggers is very low, and being able to spec more into dagger mastery through the use of runes and so increase the effectivity of dagger attack skills, such as an assassin can do, will increase your damage to a higher degree than a rit can achieve pretending to be an assassin.

So, to reiterate what all the knowledgable players above have said. If you want to play a melee go warrior and use warrior skills, if you want to play a caster go ele and use ele skills.