Omnisphere is based on samples and definitely does not have the same ability to sculpt the spectral view of a sound like IRIS can. Also you cannot import any of your own raw sample data AFAIK so that sort of limits the concept of going out and recording some interesting one shot and atmos sounds, bringing them back, loading them up and get tweaking and mangling that into an amazing sound like you can in IRIS.
Certainly not putting down Omnisphere here either. I don't have it but I know it is one hell of a synth with a massive library that covers a lot of ground and overall would be much more powerful than IRIS. But IRIS does have that special spectral ability that is basically not possible in other synths too. It is a matter of how much you want that creative potential and if you are willing to pay for it. I am!
It takes many layers of samples, effects treatments and modulation routings inside a synth like Omnisphere to create a very rich, complex and deep moving texture. But some synths like Native Prism can make the same sounds without any samples and doing it completely differently. That impresses me because it is smarter in many ways.