I actually own both libraries. Nucleus is basically a combo library that has taken essential bits from Audio Imperia’s other libraries and put them into one package. It is more minimal in it’s articulation set and mic positions, with just sustained and spiccato and “classic” or “modern” but it does include some instruments not found in Jaeger.
Jaeger on the other hand is a full “epic cinematic” library with strings, brass, percussion, vocals and sound design elements with multiple mic positions. My main problems with Jaeger is that they only include legato articulations for violins, cello, trumpet and horns. I also find the brass to be too loud and, well, brassy. There are also some recoding/editing problems across a few notes.
Overall, they’re both good for “epic” music styles, Jaeger is the better all-in-one, but I’d have to say I like Nucleus better, at least for layering. Perhaps because it’s a newer library, it has a cleaner a smoother sound to it IMO.