I don't have either but I have looked at both of them for you and there are some general things to consider. I bet you are not pushing Omnisphere to the max now so an upgrade like Omni 2 could be amazing to say the least.
On the other hand Zebra 2 looks excellent too and offers quite a few forms of synthesis. But there is no granular synth for example where as Omni 2 has got granular synthesis. I have just got into granular in a big way with both
Mangle and New Sonic Arts
Granite. I am in love with them. It can do stuff no other synth can even think about, Zebra included. But it also depends on how keen you are for granular type sounds.
(they can be musical and tuneful and also be controlled via a keyboard midi controller but mostly they are wild and out of this world!!!) I see Omni 2 offers more FM and wavetable stuff too.
But then again getting something like Zebra 2 will give you a new and fresh sonic signature synth sound. These plugins all do sound quite different today like in the old hardware days too. I love that about them. So if it is a fresh sound you are after Zebra is bound to give it to you. I use
Synthmaster for example and love it because it sounds different to other stuff I use.
Alchemy is like that as well.
I am impressed with Omnisphere myself but I also have some pretty deep and powerful machines which offer more sound variations. Everything that comes out of Omnisphere might have that sound.
(unless I am wrong and please existing Omni users put me right if I am wrong on that.) Just a few things to think about
Zo that is all.