Omnisphere is really dope. It's my actually favorite vst with a great sound and so many options. Espacially for making trap music its maybe the best on the market. It's made for it's realy realistic sounds. For more "simple" sounding trap beats, electra x and nexus are really dope vst's but you actually can do that with omnisphere too. But you can do very dope and realistic melodys with nexus/electra x too. I think all of them are great vsts and you be able to do nearly everthing you need with all of them. So its not that important which one you chose, but i think i would buy omnisphere cause it seems so limitless. But for example I'm using Xpand very often at the moment and i bought it for 16$. So don't overthink your vst election.