Domitia with costume bonus IMO.
I do agree that Seshat is still better, but only by a little bit now. The difference is small enough that I’d say variety is a bit more helpful to have.
Domitia+cb has better stats, better sniper power, and a very underrated holy defense buff that can help you sponge a Drake flails attack and still come out healthy.
Not to mention cDomitia is a 3-range offensive cleanser. Yes, Rigard is better for cleansing the entire team, but cDomitia adds raw power to your purple stack too – sometimes another snipe is all you need to kill that troublesome flank or wing.
Also, you can always use both Seshat and Domitia+cb or cDomitia + Rigard in the same team. Nothing saying you can’t. You get the best of having all the benefits of either a secondary dispeller or secondary cleanser.
Domitia’s emblem tree also happens to be one of the easiest to set, because Rogue and Ranger share the exact same attack path for both classes 
e.g. with 4* paths:
So giving emblems to either Domita+cb or cDomitia and following the path above, guarantees that both formes will have the best attack stat!
Since Domitia uses Rogue emblems (primary class) and Seshat uses Ranger emblems, that will give you the flexibility to also have two emblemed 5* dispellers. You would have to further stretch your Ranger emblems with another Seshat.
So yeah, there’s no doubt in my mind, you should do Domitia and I’d say she’s probably deserving most of your Rogue emblems too once she’s fully ascended.