I feel that Tarquin will side with himself and circumstance rather then with either, if that means having Malack and Nale work things out fine, if it means helping one against the other so be it, if it means having them both die I think he could live with that.
In answer to the question I will say he would side with Malack as the more useful of the two (both in the immediate and long term).

For Tarquin vs Xykon?
I have to go with Xykon.
While both are obviously trying to make the world a better place I feel that Xykon has demonstrated greater commitment to this noble goal, and so deserves the prize.
I also happen to find him to be an incredibly well written character who I feel could surprise me at any moment by revealing more knowledge then he is credited with without it seeming odd or forced, but also could demonstrate a lack of knowledge without it being a surprise.