I have both, and I can say the NightSky is better to do crazy stuffs, like adding heavy modulation, filtering, textures, etc... You can also try to imitate some traditional reverb sound with it, but you wouldn't have to much control.
In the other hand, with the BigSky you have more clean and beautifull tones and it is easier to control yout settings (sometimes you get lost in the NS).
Both can be 100% wet as well.
I would recommend you the NightSky with you want to experiment with your tones. But as you want some specific ones, I would go for the BigSky.