Zebra, alchemy and massive can do all the same things omnisphere does and 78% of komplete outside of kontakt and battery.
The highlight of komplete is clearly kontakt while omnisphere is essentially an advanced rompler, and in my opinion would be a much better investment than nexus.
I would recommend omnisphere for ethereal and komplete for more regular styles of music.
And depending on what you specifically like about this field I'd actually recommend reaktor/zebra.
To be sure of it yourself, please just 'demo' the audioprogram. I've demoed that thing since it was called atmosphere and only got it after the nameswitch.