They both have their pros and cons.
Doubleshots are generally considered better for the appearance of the legends on the keys, since you can have more contrast between the two colors and a more sharply defined border of the image.
PBT wears much better than ABS though, so the keycaps will retain their nice texture for longer before getting smooth and shiny. I've heard people say that they'll withstand six times as much use as ABS keys before showing obvious wear, but I'm not sure where that number originated from. The PBT dye sublimated keycaps can also have more than two colors on them, but the all of the dye has to be darker than the color of the plastic.
PBT is usually a little more expensive, and you definitely want to go with PBT for blanks, but I think doubleshot ABS makes sense for these novelty keys in particular.