Well, I've made a decision.
As awesome as Plutacra and the Driss are, I've decided to take the Thrae. Although I'm familiar with running Plutocracies, I honestly think my initial plans for the former would have been bland or unoriginal, especially in the case of the Driss. (The only interesting thing I was planning on doing was giving the Driss gurgle dialogue when not addressing another species, such as "Flamboyant Gurgling"... Not sure if these ideas were good or funny at all.)
Nevertheless, the Thrae are in more of a manageable (and more interesting) Rock in a hard place than the Driss as well; although the United principalities and the Zezhuanth are friendly, I would happen to owe the Plutacra a little, and the Theocracy won't like the deal we've made with their enemies.
Put simply, Thrae all the way.