Burrito is at 3/70.
Clarissa is at 1/1.
My other 5* Darks are Seshat and Panther both emblemed. I have Sarjana at 3/70, too. Dark is the color that this game decided I can’t get mats for, so I’m staring several months in the face before MAYBE ascending the other.
Clearly Burrito. His super bean power makes his mana very fast, which in turn makes him the go-to choice. It’s eye-watering, when you think about it.
I’d opt for variety - you already have a sniper w/ Seshat, and Panther does what only Panther can do - Clarissa may not be a direct GM clone, but I suspect she’ll be fantastic.
I am also facing this dilemma, but in my case Clarissa and Kage are the only 2 dark 5* that I have (got them both in the same Atlantis pull).
My other 5* heroes are Vela, Talli & Joon, so I have both a doter and a sniper- so should I go for Kage or Clarissa?
As you have G. Panther I would say that a maxed Kageburado would be a very fine addition to your rooster… anyway, the next 6 tabards are for Clarissa.
Go with Kageburado and use a Joon - Kageburado - Telluria - Vela - Red formation
Clarissa. Fits the DOT/AoE meta that works well with Tell, and Kage is good for damaging and not so great for finishing off.
I’d go Clarissa… especially if you typically use 4/1 or mono… that elemental link seems to be super handy.
Would love to see a rigC, tibsC, panther Clarissa, seshat (sometimes autocorrect gets me into more trouble) team. 
Related question while there’s a thread about these two heroes.
I’m thinking of retooling to a purple-Rabbit-Telly-Gravy-Alice defense. Which of the two would be better on D if neither really gets the priority for emblems?
I’m running into a similar issue in choosing who to level. Clarissa and Telly are both Paladin so they would have to share emblems.