How do they compare?
At about 140k each, it looks like a tough one to decide. But not really.
Personally, I prefer the Sinister.
You will have no shortage of mice coming at you in the coming areas, so the Attraction Bonus of the Ambrosial is not really necessary. The Shadow Mice are tough little critters, so the Sinister's Power Bonus helps put them down.
The Stale effect of the Sinister Portal Trap is also no longer a bother. In this later version of Mousehunt, you no longer toss Stale Cheese away. Instead, they become crafting items for other cheese (Ancient).
Well ok, the Ambrosial Portal IS prettier and more mouse-safe.. but come on...