You've already got a C8, so the stuff requiring larger aperture and higher magnification is covered.
So keep the focal length to a minimum and keep it portable. I'd go with the 100 and see how you get on. If you find it's insufficient, whip out the C8.
Just be prepared...at f/5 I'd expect the CA to be pretty bad, but for big faint stuff it goes against the price requirements. It'd be an interesting experiment
I hear an NP101 is a really nice scope ...
