The question maybe better phrased as 'which one should I get first?'.
They are wildly different beasts with some crossover areas. Neither can do what the other does.
With Omnisphere I can do way more than with Diva - and produce tones that I can definitely work with. It's a far deeper synth engine, but......
.....if I was shipwrecked on a desert island with a solar panel and a laptop, and I wanted something to move the soul when I played it, I think it would be Diva. It does way less than Omnisphere, but what it does is speak to me like no other software synth I own.
If I made one specific type of music that needed that Diva thing, it would be Diva all the way, rather than trying to get that essential tone from Omnisphere. If I needed to have a massive pallette of sounds to work with and a deep synth engine that could get into unique spaces, it would be Omnisphere.
I had Omnisphere first, and would probably still go that way if it was one or the other. Diva would be the luxury and Omnisphere would be the essential, but only because I don't dedicate myself entirely to that analog ambition. I've spouted off way more than is useful, probably - but it's only because you ask a TOUGH QUESTION!